Ok... my system was absolutely stable for over 3 months after i found a "solution" to the problem... So if JenBell can recollect, i was getting this on Win XP pro as well as Vista!
After way too many trials i realized that if i underclock the card to 40% of its potential and i dont use the RAM slot which is closer to my CPU, i dont get the error! i din get the error for over 3 months... but at times, the desktop wud freeze... and then get back to normal shortly.. never got a BSOD.
Then, while playing GT Legends, always noticed that on one particular track, the game wud cause the monitor to switch off!! and game wud hang... have to do a hard restart.
I thought it cud be some bug in the game causing it to crash.. but heck no! when i removed the graphics card and played with on-board card, the game worked well and i cud complete that track! :O
Then i installed Win7 beta... as soon as i logged in (ofcourse rivatuner is not installed in win7), the artifacts came up and i got the NVL error!
Then never attempted Win7... Now when i'm playing on XP, after a few minutes of gameplay, the screen wud switch off and game hangs!!
I removed the 8600GT card and tried with on-board 6150 card... all above games work well, although terribly slower!
I was convinced that its a GPU overheating problem... but now it seems it has saturated down even to the lower clocking speeds!!