RAM. Lots of RAM.
Fast hard drives, preferably a 2 or 3 disk RAID 0 for your footage, with a separate one for low-bandwidth and static files (like projects). Might as well through in another small one for the OS.
I'd be inclined to look at i7 920 (or 930 if the price difference is small) with a good cooler to overclock to 3.6GHz or more for bursts of encoding.
12GB 1600MHz RAM
I like Asus boards - P6X58D is a nice one with SATA III and USB 3.0
Velociraptor 150GB for OS drive (or SSD if you want)
500GB Samsung SpinPoint for projects and low-bandwidth files (or 1TB if you're downloading a lot of stuff)
3x 500GB (or 1TB if you're doing a lot of HD) Samsung SpinPoint F3 in RAID 0
That should sort you out.
Big question though - what software are you planning on using? However much I don't like them, I'd be inclined to drop in a GTX480 if you're using Adobe CS5 so you have the hardware-accelerated Mercury Engine for the video apps.
Photoshop doesn't use Mercury in CS5, but for everything CS3 and upwards it does make use of OpenGL acceleration so drop in a Radeon 5770 and you'll get a nice performance boost.