Pulakiyah makes very good points.

I, however, think you should go 5 boxes.
Ebay has dell gx280 s with 1gb and 3.0 processors for ~$150 - throw in another $80-100 for a 9600gt and $40(20+20) for kavoom KM and Keyclone and you have a machine ready to go with necessary software for $270-290. I got 15" monitors without a stand for $35 (included shipping - from auburntechnology - they sell govt surplus i think). Now you are around $1400 for the 4 slave machines and that leaves about $1600 for the main - good 22" monitor and quad core easy.

If you look for you local state auction house you can get the gx280 for around $100 (got mine for $100) and no shipping. Made my own monitor stand for ~$40 with parts from Lowes (required drill for a couple holes) x2.

I just think the seperate monitors makes it sooo much easier (go for 17" or bigger if you can afford it - maybe use your old machine for one of the slaves to help afford better monitors).

If you go with one computer, make sure to look into the solidstate drives for game data to help eliminate lag - this info is posted elsewhere on this board. Multiple machines spares you the lag issues too.

I think both, single and multiple, machines have their plusses. Go with what you think what you think you would like.