Here are the stats of my old system.

EVGA NFORCE 680I SLI MB REV D
D- CORE 2 QUAD 2.40GHZ Q6600 8M 1066MHZ STEP G
8GB RAM, 4x 2GB.DD2.DIMM.I
Dual GTX 465 1024 MB

It was usable for 5-boxing as long as I stayed out of SW and Dalaran. If there was something needed in town I had to move a couple at a time because of extreme lag.

I don't know much about the parts, just saying as long as the system you get is at minimum like the above you can 5-box if you put up with a few issues.