You can buy my machine which has 6 processors, 8GB of RAM and a 1TB hard drive for £355 (+screen/keyboard etc.) currently (VIBOX on ebay). Its a little noisy
It absolutely tears through footage from my 5d mk II.
You'd be fine with a Quad core or Quad core duo I'd think. The company I mentioned do custom builds or builds from their spec sheet.
You'll pay more for a laptop (obviously).