a 500 watt PSU is plenty to run your rig as spec'd including any overclocking. that PSU will NOT limit you if you decide to upgrade to any better single graphics card later. it could also handle a 7850 Xfire set up.
also BOTH PSUs have just two 8 pin PCI connections; so the corsair GS won't be able to Xfire any better than the antec HCG with the provided power connections; which also has enough amps on the 12 volt rail to support it.
so what i am saying is the GS isn't worth the extra £10
if you want to do a little reading the link i posted also have links to reviews of each PSU.
(edit: don't think a higher wattage = better. look at the 12 volt rail and the connections)