Do what JimMc said, except for motherboard pick an Intel brand motherboard with integrated graphics since SETI doesn't need  a fast graphics card. If all it will do is SETI@HOME, then pick up the cheapest 20GB or 40GB drive they carry, one stick of 256MB since you won't need any more than that. You don't need a CDR/DVD drive, just get the cheapest CD drive for installing windows you can find, or maybe even take one out of another computer and use it to install things with. The ONLY thing that really matters for SETI@HOME is cpu speed. Video card, hard drive, OS, etc. don't really play a role in how much it crunches WU's. If you price this out correctly:
CPU: $164
Motherboard: Probably around $110 for Intel with network, audio, and video
RAM: $35
HD: $20 to $30
That's $339, plus you need a case and PSU. If you can find cheaper memory, some old used 5GB HD's for sale, same motherboard I reccomended at pricegrabber.com, etc., it may be possible to build two P4 2.4C setups within $500, it'll probably go to $600, but you get twice the performance.