World of warcraft
Grand Theft Auto San Andreas
Counter Strike 1.6

The above games must be putting a bit of a strain on your card.....also you say you have updated to the most recent drivers but they might not be the best drivers for your card, try an older more stable set of drivers...sometimes older cards prefer older drivers.

Does your pc freeze when playing less taxing games?

Also, your psu is a 400w which should be more than enough for your sysytem (even most generic psu's should have no problem), but is it kicking out the juice it should be?

Check your psu voltages to see if they are well off and to eliminate the psu as being a problem.