Stumped

[Paragon09]

Okay over the past month or so ive been buying some parts for my brothers Dell Dimension 8300.

2.4ghz 400fsb
mx420
256mb ram 2700 (333mhz)
250-300wt PSU

so i upgraded him to a
6600
1.7gigs of pc2700ram
400wt psu

everything seemed fine till he his computer kept freezing, now i did a memtest86 and some HD tests everything passed. So i thought possible that it was an overheating issue since the cpu technically doesnt have a fan on it heatsink except for the huge 120mm on the side panel to it. So just another excuse to by him parts so now his computer is

AMd64 3700+
Gigabyte Mobo
6600
1.7gigs of ram
400wt psu

i install windows no skipping files everything seems fine install mobo drivers and then in a few minutes it completely crashes and continues to do so every few min i reboot the machine and once again HD and Mem test passed. It would freeze and completely lock up and then the screen would go black and go to the post blah blah everything being recogonized. So im just wondering if it would be the Video card since everything else seems to be fine with it and is the only thing that i really havent tested at all. Input would be great since i just need a few other opinions and seems like an intellegent place to ask questions. thx.

[pam123]

You've answered your own question.
Begin by swapping out the video card.