The X850 pro was a nice card, I'm still running an X800pro and have no intention of replacing till Vista and DX10 come along. Just means I have to drop the resolution on some games a bit as times move on.
To be honest, I can't see there being that much of a "real world" difference in it. Now if you could stretch to an X1900XT then I think that would be different. But you are talking a fair bit more money.
Mind you, I don't think Ebuyer has any X1800's in stock anyway. Overclockers.co.uk does, but I'm assuming your credit note ties you to Ebuyer.