Now, what will be the budget your whole system?
That will help narrow things down better.
Tom's provides a large number of articles to help system builders decide on what to buy (refer: Best CPU/GPU for the money, System Builders Marathon)
On the topic of CPU, if you are interested in an i3 then the i3-2100 would be nice. 3.1GHz, 2 cores on Sandy Bridge architecture and 4 threads; courtesy of Hyper-Threading, makes it a surprisingly effective gaming CPU to have. Biggest setback would be the inability to OC it, no biggie if you don't.
The i3-2120 increases clock speed by 200MHz with a price increase.
Then, on the lower end, if your budget is low there is the new Sandy Bridge Pentium processors. Only one worth looking at is the Pentium G860. Not much to look at there; 3.0GHz and 2 cores. Except for the much sought after sandy Bridge architecture.
On AMD's side, the one I would recommend which wouldn't break the bank is the Phenom II X4 955BE. A Deneb 3.20GHz quad-core beast which OCs ridiculously well on air, this processor packs quite the punch for it's prize category. It trades blows with the Intel's i3-2100 in terms of performance but has a slightly lower price and the advantage of four physical cores (AMD) over four logical cores (Intel).
Still need to know your budget before I can say much. Many processors out there right now.