what's your budget?
for a decent price (300 usd-ish, prob less) the 7870 is pretty beast, I have a 6950 but I think those got discontinued...
7950 is getting to the less cost efficient territory, I think, but still pretty good cost wise. wouldn't go any higher though
I'm not sure what you mean by motherboard can't run bigger cards (they all use PCIe 2 these days, right?) but if you're talking about the case then the 7950 is a bit longer (about an inch, depending on manufacturer) than the 7870, which is about the same as your 5850 now.
not that 7950 is a bad card, but if money or room in the case is a problem I would go for the 7870 instead. This is not taking Nvidia cards into account since idk much about them
good luck!
EDIT: waiting seems like the best option, yeah, but it'll always seem to be the best option, there'll always be just that one more price drop you want to wait for. You'll have to upgrade sometimes, now is as good a time as any. (Unless it isn't, in which case like, wait until you have more money or time to game or something)