If you plan to get a new monitor eventually and game at high resolutions you can get the Athlon II X3 440 + HD 5850 for under £300. http://www.ebuyer.com/product/235763http://www.ebuyer.com/product/199655
Or the AMD Athlon II X4 635, both good for gaming.
For 1440x900 the 5770 is very good even overkill and your current 8800 GT isn't bad at all at that resolution so the improvement might not be very significant. I suppose you can play pretty much all the games.
Or you can buy a monitor, say:http://www.ebuyer.com/product/158382
a HD 5770, sayhttp://www.ebuyer.com/product/189741
and the Athlon II X3 440 for £338.11.