big xp - game probs :\

This problem has plauged me for 48 hours straight now. whenever I try to run ANYTHING 3d accelerated (battlefield, the new devastation demo, NFS porsche unleashed, RtC Wolfenstein, everything lets me get a random distance into the game (for instance, wolfenstein i could go a couple of missions, battlefield is hit or miss, but like devastation kicks me almost immediately) and then just sends me to a black screen briefly, and then my computer self-reboots. Ive tried EVERYTHING i could possibly think of to get this fixed. It started when I formatted over win98 and installed windows XP pro on this machine. I have the latest mobo drivers, the latest sound and video drivers, directX 9a, even a beta AGP patch from viaarena. Ive switched out my Radeon 9100 for a Geforce 4 Ti4200, with the latest detonator drivers. no dice. Ive tried both old versions of the catalyst and detonator drivers, ive tried old via motherboard drivers, Ive tried old sound drivers, ive tried directx 8.1, ive tried removing my USB joystick, ive tried removing the sound card, like i said i switched video cards, but nothing seems to help. if anyone has any sagely input i would be almost willing to kiss you (providing it works).

my system specs are as follows.....

Athlon 1ghz
XFX via KT400 motherboard
Sapphire Tech Radeon 9100 (64mb DDR, AGP 4x)
Chaintech GeForce4 Ti4200 (128mb DDR, AGP 8x)
512MB Crucial DDR PC2100
Sound Blaster Live! 5.1
Fujitsu 8.4 gig OS harddrive
Western Digital Caviar 30 gig storage harddrive
Current OS Windows XP Professional SP1 (with evey possible update but the stoopid media 9 thinggy)
Im unsure of the powersupply size, but i dont see why it would cause a problem now, and not when I had win98.

any help would me mucho cool at this point :\

I have to say that im just a glimmer beyond extremely pissed off. The power supply rating was 250Watts. 185 peak watts. im pissed off because years ago before I knew the first thing about computers, I blew my powersupply because it was too small (250 watts). So i took it to the local shop, and they were to put in a bigger (350 watts) powersupply. im slightly pissed to learn that they just put in the same size ps that i started with >:\ good thing theyre out of buisness now or id complain

switched out the PS with one I had in another old case. I dont know if ITS size but it seems to be working a little bit better. I think its time i invest in a good top quality 400 watt powersupply. :\

thanks for the insight ryan!
im goign to run the memtest anyways Ill let you know what i come up with.

imma go find the jackass and beat him up whos with me!

well, i suppose im off to buy myself a nice new 400 watter. maybe its time to splurge and get me one of the cool ones with the blue neon lighting built in