$200 max... that was Intel's 2nd worst processor time (well I'd say worst was actually Pentium D days, Intel couldn't make a chip them if their life depended on it, so they had to use high clock speeds to fool people)
We sold an old dell with a 2.4ghz celeron, 512 ram, and a 120gb hdd for 250 on ebay profit towards my friends build.
Ending bid was $230 i think with $50 shipping costs listed but it actually only cost $22 or so to ship out.
This was about 6 or 7 months ago but i imagine that you could get at least $250 on ebay probably more along the lines of $300..especially as sad as it is to say if it is a pre-built that is known to people such as a dell dimension etc....
You are better off selling the whole thing on e-bay with no buy it now price. You might make more money with bidding. I had an old laptop I sold on e-bay I wanted $100 sold on e-bay for $250. Then I had old 32mb video card I was hoping for $25 I got almost $60 for the card.