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A quad core most likely won't benefit you right now you're interested in gaming. And no, you can't put an intel chip on an AMD board. In fact - I believe your MB is a socket 939, in which case you won't even be able to put *any* of the latest AMD processors (socket AM2/AM2+). What's your budget?
EDIT:
Video Card: http://www.newegg.com/Product/Product.aspx?Item=N82E16814150316
Motherboard: http://www.newegg.com/Product/Product.aspx?Item=N82E16813128357
Processor: http://www.newegg.com/Product/Product.aspx?Item=N82E16819115132
Total: $305, not including shipping
As long as you've got a a decent power supply, you should be good to go - your existing memory should work, though you can get a decent set of 4 GB for another $40 (DDR2 800) if that's in your budget.
Too bad you're in the UK. I have this HD2900XT just sitting around gathering dust because no one wants to purchase it from me.
i'd start with a new mousepad