Overclocking is you do to gain extra bang for the buck. It depends what you're planning to do with your system.
If you game, pick a dual core processor.
If you render or do number crunching, get a quad core processor.
I'd say you can't go that wrong with E6550/E6750/E6850 (C2D) or Q6600.
Pick any solid P35-motherboard for these and you'll be fine, for example Asus P5K-series or Gigabyte DS-serie.