I am not an expert but i belive KDE must be our best bet right now. Am i wrong?
KDE can be as light or heavy as you want. It's massively mutable, and the benefits of its' underlying architectures, I believe, are too great to leave behind.
It's just a personal opinion, but with the release of KDE 4, and a possible/likely windows native implementation comming, KDE will place itself in a position to take a lead that will take other desktop environment systems years to catch up with.
Minimalistic implementaions like Black/Flux Box and the like are simply not up to the challenges of a rich desktop implementation. Yes, I have set up Fluxbox and WOW wasn't it fast. It's easy to be fast when you have little else to do but draw a window.
There will always be ports for other window managers and desktop environments, but the depth and richness of KDE, and the size and activity of its' developers will see it the strongest base to build on for years to come.
(Now watch the Gnomers flame me to a crispy, cruchy frazzle in seconds : )