RAM problems - faulty? or not enough power?

Hey all,
Quick specs of the computer;
MOBO: SE7527rp2
CPUs: 2 x Xeon 3.6
GFX: 8800GTS
PSU: 600Watt
HDD: WD Velociraptor 300gig
OS: Windows 7 Ultimate 64bit

plus Optical drive.


Ok, so here is what is happening.
When I have 4gig of ddr2 400 RAM in the system (4 x 1gig sticks) everything runs smoothly...
However, I wanted to take advantage of the 64bit nature and increasing the RAM to 8gig.
Which I did.
Originally when I put the 8 gig in (4 x 2gig sticks) I was running Vista Ultimate 64bit.
And when I put it in and started up the computer, the computer would run for perhaps 1 minute then freeze.
There was no beeping.
I looked at the diagnostics msgs at startup and noticed that it recognized the 8gig... but it had failed.
So, I reseated it and ran it again. No luck. So i went back to the 4 gig and everything was fine.

THAT led me to believe that the 2gig sticks were faulty....

HOWEVER, I loaded W7 and thought it would be worth trying the 8gig again since maybe VISTA was the problem...
but no dice, still kept freezing up.
So I flashed the bios (to get the 8800gts compatible with the Mobo.. which worked) and tried the 8gig of ram AGAIN.
This time it WORKED... and I had no problems with the System (it worked smooth for about a month) until today.

TODAY: I had to move the HDD from a slot in the bay to a different slot, all that changed was the position of the HDD and I had a tiny tiny wee fan plugged in (4 pin molex connection).
Then when I started up the computer everything was fine... for a minute, then FREEZE.
So it's back to freezing up..

Anyway, I put the 4gig of RAM back in the system and things are working again fine.

Does having 8gig or RAM in the computer suck a lot more power from the PSU??? or does it just sound like the RAM is faulty?

All I can think is that the PSU is overloaded (although I don't see how it could be with so few drives/cards attached) or the RAM is faulty (but then it makes me wonder why it worked without problem for a month before today)...

I'm totally perplexed by it right now and I hope someone can give me some insight into what to do/try.. or just give me some general information!!

As always I appreciate any help that people can share..
Cheers!

have you tried runnning86+? http://www.memtest.org/

That may allow you to isolate the problem as RAM, versus something else..