If you have enough good parts in your old machine it may be worth going down the 'barebones' route and just getting a mobo, ram, and CPU bundle.
It depends on what you want to do with the old one, but mostly I find buying a base pc and adding the components you really want to it is a better bet than buying a full-spec monster with all the whistles and bells you don't want on it.
I've just been looking at the independant desktop PC market and it doesn't look as good as it did a few years ago, but there is some stuff out there.
This one from eBuyer seems like a good starting point with Windows bundled even.
http://www.ebuyer.com/388...m-desktop-pc-7873-0426 If you look at it like you got 4 years out of the last one if this one lasts as well that's like £150 a year. A bargain!