I highly recommend the P5b deluxe, 3 400Gb drives in raid 5, and an 80 to 120Gb disk for the OS. E6420 OC'd to 3.2Ghz grab yourself a couple gigs of ram, and you've got one speedy machine for cheap (This is a slightly scaled down version of my encoding monster, expect something like 30fps for high quality x.264 encodes.
If you're a Divx or Xvid fan, you'll be looking at perhaps 100fps or more. I haven't really tested these, because I'm really into the x.264 codec.