I've got a Radeon X800 GTO card in my desktop machine. It's an older card but it still has pretty good 3D gaming performance (it came out of the box with all pipelines enabled). I bought it a few years ago based on an ArsTechnica system building guide. I've been irritated for a while by it's poor performance in X but never bothered to look into the cause.
Yesterday I was tinkering driver problems for a Radeon 9600 HD (or some similar recent, cheap card). When using Ubuntu's fast user switching (i.e. multiple instances of X), performance goes to pot. It turns out that the driver can only use DRI and therefore acceleration on the first X instance. It's likely that the limitation will be fixed in an upcoming "radeon" or "radeonhd" driver version.
Anyhow, that tinkering session gave me a hint to my poor X800 performance. I suspected it was related to the XAA/EXA options. Sure enough, a peak at the Xorg.0.log revealed:
(==) RADEON(0): Using XAA acceleration architecture ... (II) RADEON(0): XAA Render acceleration unsupported on Radeon 9500/9700 and newer. Please use EXA instead. (II) RADEON(0): Render acceleration disabled
Adding Option "AccelMethod" "EXA" to the Device section of the xorg.conf configuration file produced a huge speedup. The performance before was not awful but the improvement was dramatic. The configuration of X has gotten vastly easier over the years due to sensible defaults but it looks like they missed this one.