Re: geforce 6600 random system lock
If you've tried everything else you could try lowering/raising AGP drive strength slightly in your BIOS settings. It has helped me with both a Fx 5200 and a Gf 6600 on two different motherboards.
Computer stuff I've owned at one time or another:
CPUs: K6-3 400, Duron 600, Thunderbird 1400, Barton 2500, Duron 1800, A-64 3700+, C2D E7200
Motherboards: Shuttle Via MVP, Asus A7V, Asus A7V133, Asus N8X-VM400, Abit NF7, Abit UL8, Asus P5Q Pro
Graphics Cards: TNT2, GF 2 MX, GF 2 Pro, GF4 MX, GF FX 5500, GF 6600, GF 7600 GT, GF 9800GT