I would just order it now man. I actually just pulled the trigger on a 4870 about 20 min ago lol. There is a promo code for newegg for the visiontek 4870 that brings it down to $265 with free shipping (I couldnt resist) I was considering a toxic 4850 from sapphire (this may be something you may want to wait for) I am unsure of pricing, but it supposedly should hit retail in the next week or so (possibly this week).
Not much has been said on pricing, but I dont think it will be much higher than that of the normal 4850. Hopefully this helps, I say go for it if you want it now and you have the money.
Just buy it now, you will be satisfied and wont regret it. I hate the waiting game, and I finally realized that it is all the same **** with very small increases in performance. If you wait until August I am sure they will announce something even cooler for September, then you will wait until September and so on. My new computer got prolonged for about 4 months because I kept playing the waiting game, and in the end when everything was out and brand new, the gains were not that large compared to if I had just bought the computer 4 months ago.