Hmm, I was under the impression that a 6870 could do much more than ~240 mhash/s. If I'm correct, a 5870 will do somewhere in the neighborhood of 300m.
The 5XXX series are very good at number crunching. And some people think that Nvidia has low numbers because it is not optimised for Nvidia, kinda like how Folding @ home works better with Nvidia, but the truth is that Nvidia sucks at number crunching