Well, I think I only upgrade / overclock because of DC projects. Heh, right now I just upgraded 10 machines and well, it pretty much doubled my output. I played around with it a little and found a combination that was not only cheap, but put out around 8 WU's a day.
Shuttle AK35GT2 KT333 Refurb from NewEgg - $40 shipped
256MB PC2100 Crucial or Samsung ~ $50 shipped from the forums
Athlon XP 1700+ Tbred from NewEgg - $60 shipped
Add this in with a GC68 from SVC for $6, overclock it to 12x150, w/ the fastest memory settings and just set the voltage to 1.7v and it's done. I didn't try and push any of the chips any further because at 12.5x150 they seemed to run into stability problems unless I used more voltage and I really didn't want to use more than 1.7v. At 1800Mhz, they easily have 3hr average WU times
So, for $150 minus $75 for whatever I could sell what was currently in the computers (Athlon 1.2/MB/SDRAM around $75) i doubled my output