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460, and id say by some margin 460's with th oc headroom are often knocking on 5850 @ stock and 5870 with good oc's, bang for buck the 460 is a no brainer out of these 2, the only other card that maybe would suit the bill is if you could find a 5850 for simular prices to a 460 1 gig.
Also the 460 gives you the option to enter 3D gaming at a later dade if you wished.
But the gain on overclocking the 5830 is not really that great, if comparing to the likes of GTX460 1GB or 6850...as the 5830 only have half the ROPs (16) of 5850/5870. Cutting back too much must have been the biggest failure on the 5830 part...you forgot that the 5800 series also overclocks well ?