What cpu?

Quad-Core is usually enough today, save you're money for when you need it.
If you're not a gamer and not doing CPU intensive tasks (eg: Video Encoding) then even a Core i3 (Dual Core, with HyperThreading) is quite snappy.

If you want 'real' snappy performance then I'd suggest getting a Samsung 830 or 840 (new firmware) SSD, or an Intel SSD for your main OS drive, it makes far more difference in response times that double the CPU clock-speed or even core count.