definetly crossfire a pair of 5970s hahahaha. love it.
i'd say the way to go is either an HD 5670, or an HD4770. i think that's in your budget, but i'm not sure what the prices on UK i don't use pounds.
5670 is a fair bit better than a 4670 and it'll give you directx11 support for the future (recommended if you have windows 7)
4770 is a bit better than a 5670, no directx11.
reason why i recommend these is in case you upgrade to a monitor with 1680 x 1050, which is the next level up from yours as far as resolutions go (if you go any big you'd have to get a different card), so i'd say that one of these 2 cards hits the soft spot.
find a sale and buy, whichever is cheaper is probably the one i'd go for.
The HD4770 is more than a bit better than the HD5670, it's approximately 25% better. When shopping for a bargain card I don't think the HD5670 makes much sense. You want as much performance as your limited budget can provide.