Re: 6600 Gaming problem :-(
Well I have the nearly same system and I upgraded to an Asus N6600 with 512Mb DDR 400Mhz Kingston and had random crashes in latest games , and it took me around a week to figure out my problem .
Total System :
Athlon XP 2000+
(Spektek) 256MB DDR 266Mhz + ( Kingston ) 512MB DDR 400Mhz
Gigabyte GA-7VKML-p ( VIA + AGP 4x)
Now I thought the crashes were related to memory , I tried taking out one stick at a time but crashes were still there . I changed VIA drivers , changed GPU drivers but still the problems were there . I thought it was heating up of my CPU since the idling temperatures were 49 degrees but nope the temperatures didn't even touch 55 and before that the games crashed . I was very confused thought about RMA but just day before yesterday I increased processor Voltage and WHOELAA , it worked . Prime 95 was stable , HL-2 ands Quake 4 ran fine .
Anyway my GPU idles at 31 / 37 which is quite lower than your GPU . I think you have a temperature problem .
Also I remember my cousin had such problems in games and the problem was the drivers of his chipset were conflicting with the GPU drivers , so uninstall them too and then see .
I have also have a friend who had the same GPU and sound card as you and the problem was his sound card drivers conflicting with the GPU drivers ( but his games were crashing not showing artifacts ) .
Try all of this , I hope it helps but I think your temperatures are quite high . Try opening one case side and install some fans , it should lower the temperatue bu 5 or 10 degrees .
Current Rig 1:
Core 2 Duo E6500 @2.93Ghz| Foxconn X38A Digital Life Motherboard| 4GB Geil DDRII @1066Mhz| Evga GTX260 | Thermaltake Armor Series Casing| OCZ 600Watt StealthXstream| Seagate 500GB SATAII 3Gbps/32Mb Cache HDD's| Viewsonic 28" LCD| Logitech X-550
Current Rig 2:
Intel E5500 @3Ghz| MSI Neo 2 FP/P35| Corsair 2GB DDRII @900Mhz| 2x ATI HD3870s Crossfire| Cooler master Casing| OCZ 600Watt Stealth X Stream| Seagate 120GB HDD| Hanns G 22" Widescreen LCD| Bose 2.1
Current Rig 3:
Intel E2180 @ 2.7Ghz*|Geforce 8600GT*| 2GB Kingston DDRII 667*| Asus P5Q Pro Motherboard|Acer 22" Widescreen LCD| Bose 2.0
*Yeah I know it sucks!