I'm working on building a rig from scratch
here is what I think i want/need so far.... obviously missing parts (ive never built a computer but know computer baisics)
AMD Athlon II X4 3 GHz Processor $75
4x 5850
PSU 1000-1200 watts
some kind of open air case to put this shit in
Forget the case. It will only cost you extra money and prevent you from cooling as much as you need to.
Also you could make do with a $20 1Ghz processor if you are only using it for mining.
Go as large as you can afford with the PSU.