I too had a similar problem with an X800GTO that would not power up, and after removing all power connections and reassembling, it worked. It seemed that the Y-splitter adapter cable thing for the gfx card needed 2 separate feeds from the power supply, as 'daisy chaining' it from one molex power connector to the next, on the same double plugged cable run, didnt work. If you have cables with a single power connector, use them all first.
I got mine working by connecting the gfx cards power cable to a cable 'run' that also goes to a dvd/cd, also a fan only power run, and other low power drawing runs, but not on a run with a hdd on it, that didnt work. (OK I still have IDE )
All said and done, maybe your supply isn't up to the task. It might be short a few watts from its name, which is quite usual if you got the supply with a low cost case.
Alot of psu's apply alot of power where its not needed, like 3 volt or 5 volt, and not the 12 V rails. Being that high wattage will NEVER be demanded from these voltages, it truly isnt what it claims. When buying a psu, always check to see how much the 12 volt combined AVAILIBLE power is. It may say 20 AMPS per rail, but still only deliver 30 amps to both at the same time. Or : 240 watts per 12V rail, with combined or 360 watts to the entire 12 volt rail. You CAN get 240watts on ONE 12V rail, leaving only 120watts for the other rail. If a cheap psu claims 400 watts, it may give 100 to the 3V and 75 to the 5V, leaving only 225 to the 12 volt rail, which drives your gfx and mobo. Always check this layout when purchasing a psu, as any decent brand WILL have it broken down in such a manner
I agree that my reccomendations above are a bit of overkill (though I like overkill so as I am not pushing the absolute limit of my equipment.)
Didja read the sticky? C'mon be honest - didja read it? In it you will find a wealth of information, including a great discussion on amps, watts, connectors, and power calculations. If ya don't wanna read it - I suggest the overkill approach.
If you want something a bit less than the FSP power supply that I listed above (which I own and love) - make sure you have at least 18Amps on a minimum of 2 12V rails (preferably 3 or 4). If you have 20 or more Amps on 2 or more rails, that would be better (notice the fine use of number theory - more is ... more)
As for quality, plan on spending at least $80 USD for a power supply. Not the perfect threshold, but high enough that the components should last you a while. Friends don't let friends buy cheep PSUs.
One alternative - is to get a drive bay power supply such as:
I'm pretty sure I read everyone's response before I posted this, but I think I may have the answer:
When connecting the video card's power (8-pin PCI-e plug), you CAN NOT have it daisy chained at all. The plug going into the card has to be a straight shot from the PSU. Otherwise, you'll get a power error. It needs to be a pure, uninterrupted source.
If you're running newer hardware, you really should have the PSU to back it all up.
If you have a video card that needs additional power and your PSU doesn't have dedicated cables/plugs for PCIe, then drop the cash and upgrade. That way, you don't have to worry about making sure it's a clean route from the PSU. That part's taken care of for you.