If you're going to run straight out VMC and not much else, I'd recommend this:
P5N-D Asus motherboard nForce 750i
Intel Quad-Core Q6600 Kentsfield 2.4GHz LGA775
2x Patriot 2GB RAM
XFX GeForce 9800 GT 512MB 256-bit PCI Ex 2.0 x16 (PVT98GYDDF)
Vista Premium or Ultimate 32-bit (your choice)
Hauppauge HVR-2250 TV Tuner (2x)
Internally, 2 WD hard drives, one for the system, other for media. A 1TB and a 500 GB like you have is fine.
This'll run 863.93 before shipping for the parts I listed here. I use them in a computer I game on, but for a VMC computer only, this will be more than adequate. I've had no problems with my system, except that using Vista 64-bit when there's no divx 64-bit. If it's sitting in the living room or something, get a HTPC/Media Center case, plenty of fans, and a DVD+/-R DL drive in case you want to read/write those as well. I'd recommend here the LG GH22LP20. It's a good drive, and a BenQ is also very nice as well. For power, get the Antec TPQ-850, it's 850W and runs beautifully.
For a sound card, get an X-Fi from Soundblaster. They do 7.1 sound and they are the best sound cards I've used yet.
As for media assist, put in the video browser by JackLuminous, as it's the one with the least complicated setup and overhead, and gives nice results with nicely sized TV and movie collections.
A straight out VMC setup not used as a main-line PC doesn't need to be as hardcore as I usually recommend for a computer with gaming, word-processing, video-editing, and media encoding duties.
Now, running this above-listed setup, your computer will definitely be fast enough to run without stripping anything from Vista. SP1 did a good job speeding things up. The Hauppauge card gives you a remote and remote receiver, so you're good there. Now all you have to do is put it together, tag your music and movies, and enjoy.
Best of luck!
James