I've seen the CPU be the bottleneck in my system while solo mining and I run a quadcore phenom II. If you are going solo I would consider getting a CPU with more cores. Don't worry about spending extra for larger CPU cache if this rig will be for mining only.
Also, at a minimum use Arctic Silver or better thermal paste for the CPU heatsink. The stock cooler and paste didn't get the job done for me (core temps exceeding 70 Deg C and emergency system shutdowns - not at all fun).