CPU: Yes you can match a 6600 with a 4400 with little problem, might be worth it if you're on a budget.
GPU: DEFO go for the 8800 GTS, its much faster than the 8600, and you wont have any problems playing todays games at max settings.
Mobo: I'd be tempted to say go with the BFG 680i, or maybe an evga one if you can fine one, both fairly similar I believe, just comes down to warranties.
The others I know little about but would say dont bother with the 975X chipset, go with the Nvidia 680 or the Intel 965 chipsets.
PSU: Either is fine, 600w is more than enough for any application atm.
GPU: DEFO go for the 8800 GTS, its much faster than the 8600, and you wont have any problems playing todays games at max settings.
Mobo: I'd be tempted to say go with the BFG 680i, or maybe an evga one if you can fine one, both fairly similar I believe, just comes down to warranties.
The others I know little about but would say dont bother with the 975X chipset, go with the Nvidia 680 or the Intel 965 chipsets.
PSU: Either is fine, 600w is more than enough for any application atm.