A price range would help.

but, I'd recommend

processor- Q6600 or higher (quad cores are great for boxing more than 3 accounts) If you go dual core just make sure it's a very fast one.
windows vista 64bit
RAM- DDR2 800 or higher 4gigs with be ok but if you want smooth than 8gigs is best
Graphics card- either Radeon 4850 or higher, or Nvidia gforce 8800 or higher. 150$ or higher is what you want.
Motherboard- Something that supports all of the above of course. Intels x38 is decent and cheap, the x48 is amazing. The p5n is decent. Plenty of nice boards out there, whatever you pick just make sure it works with everything.
Power supply- get a decent rated brand and at least 600 watts or higher.