Where have you seen the 5770 blowing away the 4870? From everything i've seen the 5770 performs on average about 10% slower than than the 4870. I believe that is largely due to it's 128 bit memory interface.
http://www.tomshardware.com/reviews/radeon-hd-5770,2446-4.html
http://www.anandtech.com/video/showdoc.aspx?i=3658&p=4
With that said, if I had to chose between the two, i'd definitely get the 5770. Even though it is slower than the 4870, the benefits of having a DX 11 card, Eyefinity, lower power consumption, etc, make it a much more attractive buy imo.
But, at this point in time, as good as the new ATI 5000 series cards are, i'd still hold off on buying them until Nvidia releases their GTX 300 series cards. I'd do this for a couple reasons: First, they could perform better. Nvidia has the benefit of releasing their product second, so they have more time to fine tune their product and make additional improvements. Second, just as we have seen intel becoming more competitive with addition/pricing of their core i5 CPU's, we may see Nvidia do they same with their GTX 300 series. And if you tie that in with my first reason, we could possibly see Nvidia release a card that is priced the same as the 5770 but performs better than the 5770 AND the 4870. And of course along with the benefits of DX 11 and the rest of the benefits of their next gen card. This not only relates to just the 5770, but the entire 5000 series. The new 5000 series cards are awesome, no doubt about it. But when it comes to ATI and Nvidia, i never commit to one or the other until i've seen what both companies have to offer.