I'll be installing Ubuntu 10.04 soon on my old Thinkpad.
I went to Thinkpad forums and some users are saying they had to use XAA!
What?!? I thought there was a transition to EXA now?
What are you supposed to use for older hardware?!? Mobility Radeon cards in R200 to R300...R250... Radeon 7500 to 9000 cards.... Mine's a R250 / 9000.
This problem/issue has been going on for eons... I remember way back reading long threads about what to edit xorg.conf to...to use XAA or EXA... what a mess... that's why I gave up on Linux on my laptop...
Now, I'm using Compiz/Effects and Debian Squeeze LXDE is running okay...but, I haven't checked out graphics effects... too scared to.
I remember having seen lots of bug reports on older radeon cards having to use XAA but I think this has been resolved in KMS/DRI2. One example is https://bugs.freedesktop.org/show_bug.cgi?id=23660. Since Ubuntu switched to KMS for radeon by default this should not be an issue.
EXA operation in the X server wasn't able to be polished until there were enough drivers implementing EXA support. Once EXA was added to a couple of drivers some work was done on the X server to improve performance, but that happened over a period of a year or so. During that time, EXA support was available in the drivers but wasn't recommended for general use at first. A bit later EXA was ok for general use but only if you had the very newest X server code.
These days EXA is enabled by default and should be fine for general use. The only reason to go back to XAA now would be if you had some quirky hardware (typically AGP bus problems) and needed to disable acceleration features in order to get stability, but I think there are EXA options that will work in most cases as well.