Just get the standard 5870 and do the overclock yourself through the ATI Overdrive utility, it is dumb easy. There is no sense in getting an overclocked ATI card unless it has better parts in it or has a better cooler.
Honestly though a small overclock on the 5870 doesn't really yield much difference in gaming. I know, I have one and have messed around with it. I usually do overclock but I have no need to do it right now. But really it is easy to OC it yourself as ATI lets you in the driver utility.