You would probably be set with a 3.2Ghz P4... There's little point in going Xeon, because it is dual-CPU capable and you wouldn't use that. I've seen some benchmarks and, if I'm not mistaken, Xeon doesn't offer that big a difference in such fields. You might try Xeon, though, if you want more memory... some Xeon mobos offer more than 4GB support (usually dual-cpu mobos...)
And on a good mobo (maybe even Canterwood), the memory performance of such a machine would be very good. So you'd probably be set with a high-performance HD array (RAID or something), a 3.2Ghz P4 on, say, P4C800 Deluxe, and 4GB of DDR400.
As I understand, Opteron also does a reasonably good job at SQL and such. It might be an interesting alternative, but bear in mind that Opteron is, at this point, brand new... I'd stick with the P4 3.2Ghz for now (which should give you respectable performance). It's just my conservative side here... But I'm just saying, hey, you could try Opteron if you wanted. It will also be more expensive, that's for sure.