One of my gentoo boxes uses KDE4, and also sees leaks with this version and XOrg 1.10.4. This is on a Geforce 6600 GT. (Specifically I am now running KDE 4.7.1, although it has done it for a while.) I haven't quantified the leak rate, but my system has 1GB of RAM, Athlon 64 3800+ but running 32-bit Gentoo (since I previously had an Athlon XP 1800+ and just moved the hard disk over wholesale.) After a matter of days (a week at the outset) X will be using well over 1GB of RAM (most will swap out, the amoutn swapped in and active will be like 40-80MB if I'm not doing much, maybe a few hundred MB if I have a bunch of firefox tabs open.) I could of course just try a newer driver -- portage has 275.28, 280.13 and 285.03.
Find attached an nvidia bug report. I notice it attached an nvidia driver install log from like 2007, but believe me that's not close to current