Well, Athlon 64 are the ones to look at really.

As for specifics, it really depends how much cash you want to throw at it.

Normally with things like CPUs, the very best are expensive, then there is a steep drop off in price and the value (i.e. speed against price) gets very good very quickly. If you can find this point and buy something around there you're laughing.

There's an overview here: http://www.scan.co.uk/tekspek/view.asp?a=30&p=3

Really it's a case of "get the best you can afford".

You'll probably most likely end up with a CPU which is socket 939 (S939) so that's the kind of motherboard you need.

You'll need to choose which motherboard chipset you want. I hear the nForce ones are pretty good, but ATI should be fine too. As I say, I've had AS Rock recommended to me as "cheaper than Asus, but still quite good".

Realistically, you need to think about how many expansion slots you want, what kind (do you want AGP, PCI Express etc.)...

Also think about the type of memory you want.

If all else fails, buy a barebones PC.