Not enough power? or vCard problem?

So my bestfriend is having this problem with his video card i will try and provide as much infomation as i can possible.
His computer:
Asus P5K-Deluxe WiFi+
Intel Core2 Duo E6700
Crucial 2GB DDR2 1066
ATi Radeon HD 2900 XT (Sapphire)
1 320GB Hard drive
1 DVDRW CDRW Combo drive
Creative Sound Blaster Live! 5.1
Antec TruePower 500 Watt
Windows Vista Home Premium

Here's the problem:
"In the beginning his computer ran fine, except in Garry's Mod for HL2 when there was slot of explosions and smoke his computer would reboot on him... Recently he got his computer shippped out to him at college from home and in the beginning it was running fine then "I've tried playing, Supreme Commander, Crysis, Team Fortress 2, and Lost Planet (All of recommended specs, you know the one the game automatically puts it to) They all crash about 2-3 minutes into gameplay. Ive even gotten flicker of the pixel crap in the Main Menu. When it would crash, it goes all "crap load of white pixels" and then a black screen. No shut down, or reboot, just a crash. I have to manually reboot" "I have the most Up to Date Drivers" " I did stress test it on benchmark, and it crashed during it (3d Mark 06)" (quoted from our conversation)

So should his card be RMAed or is his PSU not sufficent, i have pretty much the same PC build except E6750, P5K Premium WiFi+, ATi Radeon HD 2900 XT, Crucial 2GB DDR2 1066, 3 Hard drives and 2 CD drives, Creative Sound Blaster Live! 5.1, and a few more fans, but a 550 Watt PSU, are those 550watts making the differernce, Also my PSU is certified by ATi and his isnt, so what should I advise him to do next?

 

Diamond recommend using a minimum 550W power supply with these videocards.

I check a few forum and they say 550W minimun best is 750W.

 

Im not sure Clark_kent

but I am having some troubles with my diamond, I posted it in a thread not too long ago.

http://forums.majorgeeks.com/showthread.php?p=1042462#post1042462

I have a 580W Muskin enhanced <M