Project sugesstions for a new build Streaming Video/DVR system

In the process of wanting to build a dedicated computer to work like a DLink Streaming, Roku, Apple TV, etc. I will be building with old parts I have from past upgrades:

Abit UL8 MB w socket 939 AMD Athlon 64X2
4 gigs Ram
ATA Hard Drive (size still pending)
GPU-either a ATI All-In-Wonder X800xt or X1950 gt. Both are AGP.
Sound Card
300 watt PSU
Direct Line Ethernet connection

The idea is that the unit will be hooked directly to the HDTV via the DVI outputs on video.
Now before the smart remarks start, I do not have cable. Never have. TV has always been OTA. Don't want Satellite bill either. Streaming boxes limit channels. And since I already have the working parts...why not?

I am wondering what OS to use. What software will be needed? If keyboard/mouse needed will use blue tooth (have the remote control with the AIW, but really awkward to use). What is best resolution to use? TV is old RCA 52" projection HDTV compatible. I would NOT have it hooked up to the OTA, not interested in recording any OTA channels. But would like possible DVR type use. Sound would be hooked into current surround sound system from the sound card. I may or may not tie into the home network (just the i-net)

Or is all this just a waste of my time???

Any ideas or thoughts would be appreciated!!!