2 GPUs with Same Problem

[BlueNova]

Hi all

My Problem is.

I got these 2 GPU's Nvidia Geforce 9800GTX+OC 512mb & Nvidia Geforce 9800GT 1 Gig.

Both of these cards crash when i play lower ended games like Battlefield 2,CnC red alert 2 ect,after maybe 10-20 mins but when i play higher ended games like gta4 CnC red alert 3 the GPU Plays fine for hours without crashing,the only time it tends to crash on high end games is when i end 1 game and start another.also my computer's have to be rebooted,after witch all is well.

Any help would be greatly Appricated,Thx

My 1st comp specs is

Mobo Asrock Conroe 1333-DVI/H
GPU 9800GTX
CPU Intel dual core 1.6Ghz
PSU 650w
Ram 2gig
HDD 160Gig
OS Xp Pro 64Bit

2nd

Mobo Asus Phenom FX
GPU 9800GT
CPU 2.10Ghz Sempron
PSU 450w
Ram 4Gig Corsair
HDD 250Gig
OS Vista Premium 32Bit

Sorry for bad Spelling/Grammer.