[cadmium]

Lightroom and Photoshop are hungry applications. The most processor and ram you can throw at them the better. So just pick the most you can reasonably afford.

Ram is easier to upgrade later compared to the processor, but don't go under 4GB for a modern system. Anything above 3GB should get you a copy of Vista 64, which is good. CS4 and Lightroom 2 both have 64bit versions. Don't worry if you have older copies, they will run just fine too, but they won't be able to use more than 3GB of RAM.

Corei7's are the fastest out there and photoshop loves them, but they are still terribly expensive. Core2 Duos/Quads are decent and most likely in your price range. In all cases the higher the number, the faster the processor. Clock speed is more or less meaningless these days other than comparing one core2 to another core2, or a corei7 to another corei7.
It's pretty easy to tell which is which by the naming scheme:

EXXXX - Core 2 Duo (ex. E8400, E8500, etc)
QXXXX - Core 2 Quad (ex. Q9400, Q9775, etc)
9XX - Core i7 (ex. 920, 940, etc)

I probably wouldn't bother with Core2 Quads, a faster Core2 Duo will get you more bang for the buck. If you can afford a Corei7, they are the bees knees.