I've been told I'm a little slow, but I finally decided to move over to 2.6, settling on 2.6.5. Thankfully, it was painless.
nVidia drivers trivially upgraded as per instructions in the Debian packages. No 2.6.x binary modules, though.
I had to change usb-uhci to uhci-hcd and add mousedev to my /etc/modules because of changed name and changed dependencies. This recreated my /dev/input/mice, which XFree86 was complaining about.
Unrelated to the upgrade, moving my computer about slipped my CDROM IDE cable out, and I spent ages trying to figure out why KDE's service system wasn't working. It seems that it was trying to find /dev/cdrom and /dev/cdroms/cdrom1, and failing that decided to give up! Oh well, I figured that out from the log messages. Or I could have just removed them from /etc/fstab. Still quite a sad bug - I'll see if there's a bug report in the KDE bug system...
While I was upgrading to 2.6, I upgraded the rest of my software (including KDE to 3.2.2). Haven't noticed any speed/usage differences.