Bickity Bam! PreSonus just released the Firepod, a firewire interface featuring 8 XLR/TRS inputs, including 2 inputs for instruments for about $600.

http://www.musiciansfriend.com/srs7/g=rec/search/detail/base_pid/184131/

This is a great price for this many inputs, the only other good option near this range with as many inputs is the MOTU 828 MarkII. And the Firepod comes with Cubase LE which should be good enough to get you off and running, although if you get that copy of SX 2.0 that would be better.

Like I've said before, there's never been a better time for getting a firewire/USB interface, although most USB interfaces only have 2 inputs. Your computer looks like it should be up to the task of running quite a few tracks, although for top performance you'll want a second hardd rive to host your project files and audio with each hard drive as a master on its own IDE channel.

What have you got in mind for monitors? Obviously what your hearing should be accurate and uncolored.