20 vs 24 pin

You'll need a 24-pin power supply, if I recall correctly those extra for are for PCIe.

The extra four pins actually supply a second 12V rail to prevent too much amperage draw on one wire. The new spec calls for one 12V for the CPU, and the second for the motherboard/peripherals.

I think you need a 24 pin connector, however, I have seen some people say that it will work with a 20 pin connector if you don't center it but offset it to one side.

PCI-e actually uses a seperate 6-pin connector.

POST = Power On Startup Test. The beeps indicate errors. No beep no error.
Legacy= Old Technology, no longer used in new apps. or Antique.
A 24-pin power connector replaces the standard ATX 20 pin main connector. New requirements include a new power connector laid out according to the EPS specifications with 24 pins as opposed to the normal 20 pins used in the ATX specifications. 20 pin to 24 pin Adapter Cable converts your ATX 1.3 12V power supply (ATX 2.0 12V compatible) power supply to work with ATX 2.0 motherboards. ATX 2.0 motherboards come with 24 pin power supply connectors instead of 20 pin connectors. If your power supply only has 20 pins in its motherboard connector, it can be used with a newer 2.0 motherboard by just plugging it in, it will connect offset to one side, or with an adapter. They are available from many sources. I just checked a couple of sources Retail $3.99 @ Tigerdirect.com. Xoxide.com $3.99 out of stock.
You should not have to buy a new power supply(PSU).
As for the Power on No Boot problem, I need to know more info?
Have you installed the OS?
Did you use Thermal Compound on assy of the CPU?
Have you resolved the issue since you posted your question?
Can I be of further assistance ?

It's actually PowerOn Self Test.

Poiple: Are you installing this in accordance with the motherboard manual?! Prov was right - we need to know the PSU that you are using. There is a FAQ at the top of the General Motherboard forum that has troubleshooting tips for this exact type of problem. More than likely, it's either power or the mobo. For the 20/24pin issue. You can use the 20-pin in the 24pin socket. If that is a new PSU, then it should have come with an adapter - check PSU manual and contents of the box you opened. In addition to the 20/24-pin power there is a square 4pin power connector that you need to insert into a 4pinsocket right around the CPU. This is what powers the CPU and the system won't boot without this being plugged in...Are you SURE you're reading the manual....

Yes, I'm definitly following the manual. I now suspect, after some much needed rest and some more fiddling, that this is not a PSU issue. However, for the sake of continuity, and barring any objections or broken rules, I'll just continue this topic.

A brief rundown of the new components:
AMD Athlon 64 X2 4400+
BIOSTAR TForce4SLI Motherboard
GIGABYTE Geforce 7800GT
CORSAIR XMS 1GB (2 x 512MB)

I'm using this powersupply, which I've used successfully in another (20 pin connector) system before.
COOLMAX CX-550B 550W Power Supply

All of this is combined with other components that I've used before in other systems, including a DVD drive, a CD-RW drive, a floppy drive, a VGA monitor, and a hard drive.

The PSU did include a 20 to 24 pin adapter, which I've now plugged into the system. I've unplugged all of the components (including power and data connectors), except the hard drive, the optical drives, the motherboard, CPU, and the GPU. I've also tried putting a different power supply in there, (X-Connect™ 400W ATX PSU). Yes, the square 4-pin connector near the CPU is plugged in. The GPU is plugged into its 6pin connector, via an adapter that was included in the box. I've also reduced the ram down to one stick (plugged into the slot labeled DIMM1), and swapped that stick out with the others. I've also tried clearing the CMOS (per the manual instructions).

In all cases, the system powers on, including the fans on all the components. The hard drive spins, and the LED indicator shows HD access. There are no beeps, which is not a situation mentioned in the manual (i.e. "One long beep followed by two short beeps" indicates "Video card not found or video card memory bad". Or, "One short beep when system boots-up" indicates "No error found during POST".) There are not beeps at system start up, or otherwise. During all of this, the monitor does not look like it is accessed. The led blinks yellow slowly, which seems to correspond to standby mode (a green LED indicates on, which is what it normally switches to right at system startup). I used the standard thermal paste that was included with the retail CPU/heatsink, which was already on the heatsink when I opened the box.

This is the first computer which I've built which has not worked at all. Any help, please?

I ran into this recently and the problem was the BIOS flaked out. Does your mobo have redundant BIOS? if so, try following the steps in your manual to restore from the backup BIOS.

As far as I can tell, no. There's no mention of such a thing in the manual, and the motherboard isn't even listed on their website. I'm going to try changing out the vid card. *off to Circuit City, with their nice return policies*

Are there any unused mobo mounting stanchions? This could also be a case of shorting on the mobo. You can try with the mobo out of the case on a nonconductive material (phonebook works). Short the 2 pins for the power swtch with a screwdriver to start the system.

ya man 24 pins. I have the ANTEC True Power 2. 550w. Works great underneath my mobo in my ANTEC P180  

My first Build of my 939 Asus Sli Deluxe provided me witht eh same problem.. my Enermax 460 watt PSU had the wrong connector for MB power.. the answer >? an Adapter that takes it fr0m 20 to 24 pins.. Free from my local computer guy Cause they Rock !!!

As far as the rest of your problems it still does smak of a power issue.. but im wondering.. did you actually power the system up with only the 20 pin plugged in ?? and i would definatly take each piece of hardware the system is built with and test in a "running" computer . if you dont have one.. the shop i deal with would do it for me..

antecs usually perform great.

Is the system running but not displaying on the monitor? Vid card seated properly?

Yeh, Great Performers This is the same problem I had with Antec back around 1999-2002 where I went on a rant against them. 200W Gateway power supplies (Newton Power) were stable under greater load than 250W Antec's back then.

Yeah the 20 - 24 pin conversion can be difficult, I was always using the 300w or so power supply I got a new MSI 965 NeoF V2 and it has the new 24 pin deal. I have heard that oh the extra 4 powers the SATA, oh the Extra 4 powers the PCIE... Engineers love to change things, - quote from one of them star trek movies.

have been running my system with the 20 pin connector for about 6 months now. Always noticed something strange, my computer will turn on , and turns on and off about three times before it finally catches and stays on.
This is probably do to my 300W power supply , when reading the user guide for my board I noticed it has a minimum requirement of 350W. But it works..

Going to be getting a new power supply this next week during the first, something to tell everyone "THERE IS NO SUCH THING AS A CHEAP POWER SUPPLY!" -Unless you want trouble! Basically stay the Heck away from anything priced under oh say, $50, some of the best ones that I have seen are..

OCZ, Fortron, Corsair, Antec

Some of the worst I have head tell of are ChiefMax sold by 3btec, heard one fellow say he plugged his in and it exploded in a bright shower of sparks. Another said he got a 650W and it blew out after guarantee was up, he opened it up to find the transformer said 400W.

Oh a number of years ago I was working at a store and they kept bringing in dead computers with "DEER" PSU's must have been a bad run or something all in a couple of months, saw more of them then any other power supply out there..

think of it as , you wouldn't buy a new Cadillac and put a "Four banger engine" in it would you?

have a good one folks..

20 pins will work on certain mobos with 24 sockets, depends on which ones the mobo relies on I believe... just get an adaptor from a local retailer. If you don't have that extra 4 pins, it kind of says that your mobo is too good for your psu  

And we have a recent record

2005!!!

*Blows off the dust*

*Lights a match*

*Burns reply button*