Yes SLI and Crossfire show only a tiny increase in performance on the usual resolutions..
I bought a 7900GT TOP by Asus, and the benchmarks looked really promising.. but I'm afraid that card is a bit above your budget.
How much do the 7800GT's cost nowadays? Maybe that's an idea?
EDIT:
Oooh wait! Great Britain Pounds! I thought it said dollars or something. Well, since 250 GBP is 370 euros, you should be able to get one, I suppose