J, I have looked at the benchmarks, and obviously more than you, and if you think you can simply make up for it by adding a second HD5770, means you DEFINITELY didn't read enough of the reviews showing the limiting effect on higher resolution compared to the HD5850 especially when it comes to min fps and having smoother performance and not just boosting their easy parts giving the impression of equal avg performance. Running on two HD5770 means that's it for scaling, anything beyond that is limited, and also far from guaranteed, whereas the HD5850 will always perform like an HD5850, not drop 50% because a profile isn't support in drivers, or a game doesn't support Xfire or SLi without a hot-fix which may or may not come.
The HD5770 is a right now solution, and factoring in a second HD5770 at launch makes it a forgone wasted path. If all he ever needed was an HD5770 and was satisfied with turning down the sliders in games to accommodate the lower power, then that would be fine, but if he's looking for a consistently good experience from the start, and with growth into the tougher games that are already out (heck Crysis at enthusiast already chokes the HD5770 even in Xfire) , then the HD5850 is the better choice and with more upgrade options as well as more features too.