http://msdn2.microsoft.com/en-us/netframework/aa497338.aspx
256MB of RAM and a “high-end” P3 are pretty cheap nowadays.
The biggest issue you might have is just compile times. I doubt you will be writing anything complex enough to cause a problem. eBay is the way to go, as people in buffalo seem to think that crap is worth something. Occasionally you might find something at the fleamarket. But for the $20-40 price range at most you’ll probably end up with a 533 or something lower.
If you are REALLY cheap scrapping one together from garbage also works. Usually not too hard to find a case/power supply or even whole computer. As computer noobs typically toss them out for easily fixed issues in nicer neighborhoods.