535 watts is plenty, and that psu has 2 pci-e 6 pin power plugs on it. The 4850 needs only one, so it should work just fine.
The one problem I see is that the entire system might bottleneck the performance of your 4850, but that would require getting an entirely new system, which I'm sure you don't want to do. Perhaps try overclocking your cpu to increase performance.
------------------------------Core i7 920 @ 4GHz under a lapped TRUE, EVGA X58 3xSLI, Three EVGA GTX 260/c.216 in Tri-SLI, Corsair Dominator 6GB DDR3 1600, Intel X25-M 80GB ssd, WD Caviar Black 1TB, Caviar SE 640GB, Silverstone Decathlon 1000W PSU, Lian-Li Armorsuit PC-P60
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