Whatever someone's willing to pay! (sorry, my idea of a joke).
It's difficult to say really, especially as the dollar is my country is worth a lot less than the pound.
I'd say 200-300 pounds, but that's just a wild estimate, could be more. Add 40 pounds if you have the XP disk, people like that sort of thing. With a new graphics card that would be a tidy computer. If it takes PCI-E cards that is...
Have a look at this ebay link (and this one ). It's good for a laugh, these guys are such scammers! They say it's a 4GHz processor because it's a dual core 2GHz!!!!! I can't belive ebay let them get away with lying so blatently.
They've sold 2 of one, and 1 of the other, so people are willing to pay that price (although they propably think they're getting a 4GHz beast), so it gives you a rough guide.
EDIT: Those ebay ones have no monitor, mouse, keyboard etc.