Here is a suggested build I put together, its $600 and it comes packed with power.
Triple core CPU, runs any media with no problems
HD 5570, runs HD video, rendering, photos, and even light gaming with no issues, plus it has an HDMI port for HD out.
Small case, perfect for an Home Theater/MEDIA PC.
Blu-ray drive, which you can drop for a DVD drive if you dont want to spend as much.