Personally I use one "super".

Specs are:

Quad core x9300 3.0ghz
Dual nvida 880gt cards
4 gigs ram
1tb drive (used for a lot of other things of course).

Runs very very well. It's currently at the shop for the moment, so my backup is:

Dual core 3.6ghz
Single nvidia card.
2 gigs ram.

Runs ok except when I'm in capital cities where it struggles and I dead with fps of 15-20.