You will of course see higher framerates with xfire/sli but decision is yours on whether it's worth a more expensive motherboard, new powersupply and an extra £200 on a graphics card. Which MB's were you looking at?
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I was thinking of getting the gigabyte-p67A-ud4, but now i realise the UD3 also has 2 pciex16 ports, and i also found out that x4 and x8 make almost no difference in performance
Msi g45 is on today's deal for £88.79 which is another alternative to the UD3.
£7 more will get you a GD-53 which you can sli if intending on buying Nvidia cards. Both are excellent deals for the price.