Depends on what SDK you're using. Do you have 2.1? 2.5? or something newer like 2.6?
2.1 does best with worksize of 128 and a lower memory, like 170. 2.5 does better with worksize of 256, but a memory of 300. 2.6 or newer should not be used on a 5xxx card.
Now I was using CGMiner, but I ran my 5870 with SDK 2.5 at core:980 mem:300 kernel:phatk worksize:256 vectors:2 for ~460MH/s.
not true, 256 + underclocking gives better hashrates even on 2.1 which I am running right now