Don't wait, Windows 7 is Windows Vista with some new cosmetics and a few added features - nothing has really changed from a technical standpoint that would result in the same problems as going from XP to Vista did.
The architecture is mostly the same - think of Vista to Windows 7 like going from Windows 2000 to XP.
You will most likely have some problem going from XP to Vista OR Windows 7 - none of them will be a better path from a compatibility stand-point. If you didn't go for Vista before you will have to handle the problems you might have had then, now, instead. Regardless of whether you choose Vista or Windows 7 or wait another six months...
...also, Server 2008 is Vista SP1 - just a bunch more services disabled by default - so the "Vista sucked" thing is something I never understood. I think that was just bad luck and too much bad word of mouth - it didn't actually suck at all ^^ (oh well, Aero Glass was and still is rather ugly)