it won't be a 3d desktop in that sense.
People always get confused about that, though it'll have the capability for some things to be.
At the moment, it uses 2D hardware using Direct X in software.
Though in longhorn, it'll use Direct 3d hardware to make all the windows, etc, so it should run faster on good cards, and better cards will have the capability to run more effects, which its the processor really that defines if you can now.
I have a feeling it'll come out in 2007, so still a long time yet