I run an q6600 quad, 780i, 4gb ram, and my trusty old 7800gtx. I can 5 box with 20fps on alts, and 60+ on the main. My bottleneck was video settings.. I turned down everything I didn't REALLY need, and it made a difference like night and day.

And if you want to know what I started 5 boxing on? An AMD 3000+ Venice.. that's 1.8ghz, 2gb ram, and a 7800gtx. I made 10fps alts, and 20(outside of cities) on the main. Didn't even bother trying to walk into a main city with all 5. Had to go one at a time. And I made it to 30 on my druids with that setup.

With parts prices nowadays.. I bought my q6600, evga 780i, and 4gb ram for 540 bucks out the door. Added my old case, hdd, 7800gtx, power supply, etc etc.. and I can run my 5 druids, plus my main without a sweat. Can even run in and out of Orgrimmar as I please!