this is the setup i use for 5boxing

Intel Core 2 Duo 644 @ 2.13GHZ
3.25 MB Ram (4gig but only goes to 3.25 due to 32bit OS)
NVIDIA 7800 GEFORCE graphix

not exactly top of the line, in fact everything is over 1 year old except the RAM. I get about 30fps in org and all around azeroth so far (didnt take it to outlands yet so i cant comment on that) which is enough for me.

So it doesnt take the best machine in the world to handle 5 boxes on 1 computer, but i would recommend a dual core (core 2 duo or AMD equivalent) at the minimum. NVIDIA 7800 handles 5boxes surprisingly well but everyone here swears by the 8800... apparently its got the best performance/cost ratio =]