I'd wait for Fermi, it's either going to beat the 5000 series cheaper (though so far that's not the case) or match the 5000 series (seems to be the actual case) which may cause ATi's prices to drop, either way your either gonna have Fermi, or a drop in price on the HD5850/70
That said, bear in mind that it is highly unlikely that Fermi will turn out to truly beat the 5000 series in anything except for nVidia specific technologies, and due to more VRAM, very high resolutions (like 2560x1440), and at the same time the 5000 series has been out for a while, so problems you might have, have already been solved, there's tons of support and experience out there and there are a couple cool special editions out now as well with better cooling out of the box, we also already know that they are incredibly overclockable, it seems like nothing to get these guys to 1000/1200, sometimes on stock cooling.
If it were me I'd get the 5000 series but I'd wait until Fermi was released, just to see if that slim chance turns out to be right that the prices will drop.
Finally, think about it, Fermi has had some rushed production here at the end, I am a little leary about their reliability and potential problems here in the get-go, I guess only time will tell.