basically what your gonna wanna do is get a nice system where the parts complement each other, then spend a little extra money on a Sound-card with MIDI ports, Optical lines, HDMI, headphone amp etc, also with the ability to link to video cards via PCI slot.
you don't really need a lot of computing horse power to render audio. even multichannel audio mixing programs. I got a nice little set up that runs mixcraft 5 flawlessly with a yamaha p-81 digital piano. (full set weighted keys) and it runs it perfect with a USB midi port. i can also set up a music hub to plug in Guitars and mics etc.
check out my profile for my system specs. I put it together for about $1200 almost 2 years ago. now you could get the same system with better parts for easy 500 dollars cheaper.