most of the people on the kde thread seem to think it is a bit heavy. There are some advantages to installing kde, though. Mainly the ability to run a recent konqueror in a another wm (like xfwm, or the default wm), make use of kde apps like kate and konsole in the same way. Some of this would be true for gnome, as well, of course.
kde looks fine on the z, and is useable, though it lags a little sometimes.
Re gcc4 read some tests run by someone on the project (I think..it was a while ago and I was actually looking for somthing else at the time) I got the impression that it won't make anything faster or smaller.