At the moment, I think, the fastest processors that you can get are the new Intel Core 2 Xenons (think there have the prefix "woodcrest" or something like that.)

You can get a dual socket motherboard that runs these.

If your on maya 8 will you be running 64bit or 32? Just that you can add more RAM to the 64 bit system (think that the tops for the 32 bit is 4GB but might be wrong) Then again what you need to workstation for will determine how much you want to spend.

As for a graphics card I would go with Nvidia as ATI sometimens seem to have some problems even though they are qualified (just by looking at other forums and the posts there) and go for a Quadro card if possible as these run smooth as butter, where sometimes the Gforces can be a bit funny depending on the drivers that are installed on them.

I cant really comment on a desktop spec as i've jsut bought a laptop to do my work on but heres the specs that I've got.

Intel Core 2 duo 2.26 GHz
2 GB DDR 2 Ram
120 GB HDD
Nvidia Gforce 7900 GTX 512 DDR3

Pretty damm fast for a laptop!

Good luck