That Tom's hardware chart is a bit off. I get 370MH/s on my 7870 at stock, 400+ overclocked.
what are your overclocking settings?
Can mine 24/7 (although it certainly isn't stable for gaming) at 1100MHz core and 950MHz on the ram (I'd turn the RAM down more if it was allowed by the drivers) using cat 13.1 on linux. Mining with poclbm using -w128 (no vectors, it slows it down for me). No changes to the voltage. With -f10 that gets me about 407MH/s