That setup will make a nice gaming HTPC. As for the sound, will you be running a receiver?
There really isn't a "conversion" from a gaming PC to a HTPC, the main difference is if you use media software like XBMC, etc..
Equipment - 47" Vizio 120hz 1080p SV470XVT - Onkyo TX-NR809 - Pioneer BDP-150 - HTPC, Xbox 360, Wii - Turtle Beach X41 7.1 Headset - Sennheiser RS170 Headphones
Speakers - Polk Audio CS2, Monitor 70's, Monitor 30's, BIC F12 12" Sub
HTPC - Intel Core i3-3220, 8GB DDR3 1600, 560ti 1GB, 7TB of HDD's, LG Bluray