Problem with may New Video Card

PC Specs

Intel E280 Processor
Asus P5SD2-VM MOBO
1GB RAM
160 GB Seagate HDD
500w PSU

WinXP SP2

TDK DVD+-R
Samsung Super-Multi

LG W1942T LCD Monitor
1440 x 990 RES

Hi I bought a PC few months ago with that specs, as i want to improve its performance a purchased a 8400GT Nvidia Card which is 512 MB a month after i bought the computer
But then, the color and graphics and my desktop improved but whenever i play a video and play games like rocket mania, secret of margrave manor etc. (popcap games) and games like nfs underground and GTA San Andreas...
my system hangs and then reboots, sometimes the blue screen of death appears, so i decided to return the VC i purchased, after trying for four time (getting it replaced with brand new same make same model) the condition has not improved
then i decided to purchase a different model this time a 8500GT which has 512MB and 128 Bit.

and yet nothing has improve, still cant play games which needs hardware accelaration etc...

what could be the probleM?

thanks for helping....