Whatever you get, I would definitely look at EVGA for your mobo and video card. They have a lifetime warranty on all of their components, as well as a trade up program.
My current rig:
Intel Quad core Q9650 OC'd stable at 3.6ghz
EVGA 780i SLI mobo.
4GB Corsair Dominator PC2 8500 ram
2 WD RE drives in Raid 0 as well as 2 backup data drives.
EVGA GTX 285 Superclocked
BFG 800w PSU
As far as the video card goes, only get one that your processor can keep up with, or you will just be wasting your money. My E6700 C2D that was overclocked to 3.0ghz and was still a bottleneck for my GTX 285, that's why I upgraded to the Q9650, which IIRC is the fastest LGA 775 chip you can buy. The difference was huge in my FPS in games like COD5 and TF2.
The C2D processors have become very affordable since i7 made its debut. I read at Anandtech that Intel will be coming out with another generation of processor early next year IIRC, so rather than upgrading my whole system, I just stuck with the LGA 775 chipset and upgraded my processor.
Also, from all of the benchmarks I have seen as of late, quad core really doesn't gain you much in games, but is a huge help in things like video/audio encoding.
Go to
Toms Hardware to see some extremly good benchmarks on basically any type of hardware, they will give you a good idea of performance. They also have guides for building PC's in different price ranges.