For your RAM make sure it's at least PC6400 (DDR2-800). Your Gigabyte front side bus is 1333 or 1600; its memory standard is DDR2-1066 or DDR2-1200, but I'm leary of those because if I recall correctly there were never standards established for the speeds in DDR2. Most of the RAM certified by Gigabyte is DDR2-800; DDR2-1066 or -1200 would give you a bit more head room, but for my money good DDR2-800 is a better buy. If you go below DDR2-800 you'll be losing a few percent in potential.
I used the Q6600 in my last build, but you may be able to find a Yorktown quad for about the same money. They offer better performance at the same clock speeds and more headroom if you ever do overclock. In theory they are cheaper per GHz, but in practice they are more expensive because they're better and use less energy and everyone wants them. It's hard to predict the best CPU for games two or three years down the road; if performance remains weighted toward MHz, then an E6750 or E6850 (or even better, and E8400 or E8500) would be best. I'm betting developers will get better at programming parallel threads since consoles are more popular and console code must be strongly parallel to give great performance and great graphics, so I bet quad core myself.
I too prefer Western Digital hard drives to Maxtor, but I've used plenty of Maxtor drives with no problem. In fact, I've not personally had a hard drive failure in less than three years for probably a decade. I can't remember ever losing any IDE or SATA drives, even among our work computers. The only hard drive failures I've had personally have been Seagate, but that's back in the day of RLL and MFM drives, when MTBF ran around 30,000 hours except for Quantum. (I once ordered a 105MB Quantum hard drive, and the guy told me he had just sold six of the same drive to Jerry Garcia of the Grateful Dead. Who'd have thought the lead man of the Dead would be a computer geek?) The only IDE or SATA hard drive failures I've seen have been Seagate, but they've been in low-end family computers - not a good comparison, since the vast majority of low-end OEM machines use Seagate drives and that class of computer tends to get knocked around. Most business machines I've dealt with have used Seagate drives as well, and I can't remember a drive failure in one of those less than three years old.
You'll love the 8800GT, it's everything the 9600GT is and more. Brands such as XFX, EVGA, and BFG (I think) also offer lifetime warranties, which might be worth a few more bucks even on a limited budget.
