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I would forget the SLI option, your core2quad at 3.5Ghz will be a bottleneck, stick to single GPU's.if it's the spec in your sig the mobo only does xfire.
480 or 5870. The 480 is a lot hotter then the 5870. And does not give you a lot more frames.
Personally would avoid dual cards whenever presented with the option to.Do you have to pay VAT?
If not this comes out under £400
Cost wise it works out the same as 2 of the 470s mentioned earlier, except it is a single card. And you get some 'free' stuff.