My computer has 2x1 gig sticks of ADATA PC6400 RAM in it and I wanted to add 2 more to play AOC without a hitch. I ordered 2 gigs from new egg and it came yesterday. Problem is, I installed them in my P5N-E SLI Mobo and I turned it on and My monitor wont come on and it doesnt seem to boot. I put only the new ram in it and it boots fine.. I mix and matches different configs of only 3 sticks in it and My OS recognizes 3 gigs in it. I thought it was my XP at first so I loaded up Vista and ther same thing. I add that 4th stick and it wont boot up... the machine turns on but the monitor just stays idle....... Any ideas?    
 
I have a core 2 duo 2.66, the above Mobo, a 9600 gt card and a 600 watt coolermaster SLI power supply.  

In BIOS manually set the voltage to 2.0-2.1v for PC6400.  Also, adjust the timings manually to 5-5-5-15.  You may have to increase NB voltage also.

Agreed, them baddies starving for some juice!

On the face of it, this sounds like a bad motherboard. Does the failure only come when a particular slot is used, or does it work with any combination for slots while using three sticks, but adding the forth kills it? Also, it the Vista 32 bit or 64 bit? If its Vista 32 bit, it will have the same problem with recognizing ram as XP does. Try testing the ram with Memtest86 as well, as if there are errors, they may not be enough to push the computer over the edge until the forth stick is added. A last thing to try is increasing the voltage to the ram. I've had occasion where I needed to add some ram voltage to run stable under Prime95, but my computer still booted up adn ran fine in normal use even then, rather than refusing to start.
 
A last thing I can think of is that the PSU might not be putting out enough power for the forth stick. The rating of the PSU is high enough for the majority of computers, but it might not be putting out its rated power. A PSU problem is a long shot, but had to be mentioned. My main thought remains that there is a motherboard problem, though.

I have the PC6400 Adata.  Although it is rated at 1.8v, I have to increase the voltage to the max my micro board will do, 1.9v to run 4 DIMMS stable.

LOL.  I also have a MB that will not run 4 DIMMS no way, no how.  Bad MB.

Adata timing are already 5-5-5-15. And vista will recognize 4 gigs with no problem.

 
OP's system will not boot with 4 gigs installed.  VISTA recognizing 4 Gbs has nothing to do with his problem.  Read OPs post.

OP has determioned all 4 DIMMS are good.  His only option is to adjust the voltage(s) manually and try and get the system to boot 4 gigs.  If that doesn't work, like Sailer suggested, OP may very well have a faulty MB.

 
This was in response to sailors post. About vista not recognizing ram, not the ops

Ok, yeah I agree with sailer.

I run xp 32bit, and vista 32bit, both have seen and recognize the 4 gigs i have installed, so mebbe i just lucked out then.

Vista 32 will recognize 4 Gb when SP1 has ben installed.  Any 32 bit OS will not appropriate 4 Gb system RAM depending on other system components like a video card taking up some of that 4GB system space.

Vista 32bit will recognize 4GB of RAM exactly the same way whether you have SP1 installed or not.  The only difference is what's displayed in the System form.  It's a reporting change only.
 
http://support.microsoft.com/kb/946003
 
Clint

Yep thats true, it wont use it, with vista though, it will recognize the 4 gigs, and report it as you said. Currently my vista and xp both utilize 3.5 gigs. I just dont understand how this could be a problem for the op's computer not booting up with all 4 gigs installed.

 
The heading of the link states what I said, with SP1 installed VISTA 32 will show 4GBs system RAM.  This is just a cosmetic change, Vista 32 still appropriates system RAM as it always has and as all 32 bit systems do.  The typical Vista 32 system with 4 Gbs installed has 3.1-3.4 Gbs appropriated depending on other system hardware like a video card taking up system address space with 512 MB Video RAM detected by the OS.  You must have SP1 installed on Vista 32 to show 4 GB RAM if that's what you have installed.