I just ordered a new computer that should be very capable of 5-boxing and it cost me less than $1800, including shipping & assembly.

New computer has been purchased - specs, rationale, and thanks

Highlights:
i7 920 proccessor
12 GB of RAM, supposedly upgradeable to 24GB
GTX260 video card, not the top of the line, but I'd say easily in the top 20%
SSD drive, could make the biggest difference for 5-boxing if the read times are virtually eliminated

I would highly suggest using www.buyxg.com. Their prices are comparable to ordering individual parts on www.newegg.com but they put it together and offer warranties.

Good luck! If you'd like to wait 3 weeks until I get my new computer then I'll give a more detailed review.