To 3 box..

Intel Core 2 Duo
2gb Ram
8600GTS (I have 2)

I rock that and I can Quad box without problems, a little lag when loading the game and in between zones but thats it.
You can build that for cheap now. $500 without a PWR Supply. $600with.


Now here is a suggestion if you want to go big.
Quoted

Wilbur:
Q6600 - Recommended Processor
4GB DDR2 PC6400 - Recommended RAM
8800GT 512MB(+) - Recommended Graphics Card
Good brand 500(+)watt Power supply.

Without the above you'll only be gimping yourself if you want to do a 5 in 1.