for a PC with that capability, all you need is a basic PC, anything from celeron to pentium 4, sempron to athlon, with a 7.1 CH sound card, and 7.1 CH speakers.
The PC shouldnt cost that much, maybe $1000 tops, but the sound card could be $150 approx.
A touch-screen monitor can be expensive, depending if LCD is necessary...
Check these sites for deals:
Ebay - http://www.ebay.com
Newegg - http://www.newegg.com
TD - http://www.tigerdirect.com