Using lightroom quad cores help - ALOT.
Quad core downstairs at Q6600 2.4ghz beats my E6600 @ 3.2Ghz easy.
It also has double the L2 Cache at 8Mb over 4Mb.
If your looking for heavy lifting on the cheap.
Q6600 G0 Stepping
Good Asus Mobo (any £90+ ones should be hot to trot)
4GB (2x2GB) DDR 2- 800 CL4 or DDR 1600 if you can splash on it.
GeForce 9800GTX (well really anything from a 8800GT upwards should be more than enough)
Some decent hard drives, theres not much in it really.
Great setup, very similar to mine. If you build it yourself you should not have to spend more than 600 bucks, less if you use your old case, DVD drive, power supply etc.
The q6600 is a great quadcore processor, but make sure you get the G0 stepping (codename: SLACR). It runs less hot, uses less electricity, and provides great potential for overclocking.
Mine runs at 3 GHZ with the default cooler. That's quite a performance increase, for free!