Dell Optiplex GX620 USFF. Very small. Very quiet. Handles HD flawlessly. Integrated Gigabit.
I have one on four of my HDTVs. Very slick.
I paid under $300 each on ebay. Just have to be patient.
Here's an example:
http://cgi.ebay.com/ws/eBayISAPI.dll...m=110112442735
Tom