There are a couple of trade-offs you might not be aware of.

* Given a choice between a faster processor and more memory, more memory is always the better choice.
* Likewise, a choice between a faster processor and a larger hard disk should always go to the larger disk.
* It doesn't sound like you'll need it, but good 3D games performance depends a lot on the processor as well as the graphics chip. It's been a while since I've looked at this, but AFAIK, a moderate graphics chip will perform better with an above average processor than a high-performance graphics chip with an average processor.
* Jack's already mentioned the trick of buying on the "knee" of the price curve.
* If you're going to build one from bits, buy a brand-name case! (The cheap cases really are harder to work in.)

Wade.