XFCE will run faster than Gnome and it has more of a Mac OS feel without adding anything to it. It has bottom of the screen deck like Mac OS X with an autohide option. LXDE is lighter though and will be more responsive on old machines. If you have a G3 iBook I'll stick to LXDE.
Has anybody tried XFCE on MintPPC? I have Xubuntu Oneiric running on my old Dell C400 just fine. It's my daily work computer works fine for almost everything. I even had QGis running on it for a while, that was a bit slow but still workable.
I tried XFCE on top of MintPPC but i didn't find it that much of an improvment, not even esthetically. The Mint panel has also an option to autoappear/disappear (even better than those of Xfce, i find). And then, i found this tip
which really makes a lot of difference esthetically and nearly costs nothing performancewise . . .
pcmanfm is highly adaptable to do things nearly like the finder (but giving you a lot more of control).