What ever you buy you'll max it out in about 2 weeks. Make sure you get a shed load of RAM (1Gb or more helps) as much processor as you can get your hands on too.

The only thing to do is to work out whether you'd be best off blowing all your money on the computer or wheter you should get some DSP cards to go along with it.

I'm a bit of a fan of DSP based stuff, Access Virus plugins and the Sony Oxford EQ on the powercore and the Universal Audio compressor emulations on the UAD-1 are well worth having.