9800 gt problems

i just bought a new pc one month ago. My specs are:

PHENOM Triple Core 8450-HD8450WCGHBOX
MB AM2+, AMD 770, MSI K9A2 Neo-F
GeForce 9800GT, N98TOC-1GH (700Mhz) 1Gb DDR3 (1800Mhz, 256bit) Gigabyte
2 X HDD 80 GB WD800AAJS, Serial ATA2, 7200rpm
2 X DDR2 1GB, PC8500, 1066MHz, KINGMAX
Case Middle 450W, FRONTIER Irock 12cm fan/front, 8cm fan/rear, airduct

The problem is that when I play games like Crysis, GTA 4 or GRID at high details and antialiasing, sometimes the game just freezes. When this happens I either get an error message like "The display driver has stopped working..." (and I get those rainbow colors), or the game just stucks and I'm forced to reboot the system. I've tried reinstalling the latest drivers or old ones and it's the same problem. When I adjust the settings, or when i remove the antialiasing of the game it works fine. I tried some programs to see the temp of the graphic card (nTune, speedfan) but I don't see any temperature. I don't think it's a cooling problem because when i touch the card it's just a little warm.(my old card was a 7600gs and that one was hot as hell but still didn't caused any problmes). Can anyone help me?