40Gb HDD (boot drive)
80Gb HDD (video only drive)
Graphics card with TV out
But budget allowing I would go up to:
Fastest CPU you can
1Gb or more of RAM
Larger HDD's as required
If you have analogue footage you want to capture/edit then a dedicated internal analogue capture card, or external analogue to DV converter.
Double headed graphics card
Obviously plus all the other required hardware like monitor, sound etc. I would steer well clear of onboard graphics, but onboard sound should be good enough.
You should be able to get the lower spec PC I listed for around £500. The better specs should only cost another £200 ish. Of course you could start to look for things like real time effects etc on the capture card with hardware codecs etc and spend a couple of grand on that alone