That is a different package/driver than the 173 and doesn't support the
upgrade xserver:
http://packages.debian.org/sid/nvidia-glx-legacy-71xx
and is the wrong driver for your card.
Sorry I don't know what I was thinking. I was obviously using the nouveau driver. Previously I'd been using the proprietary driver, but it must have been the 173 not the 71 variety. It was all working fine, after all, until the upgrade to xserver.
More:
http://packages.debian.org/search?searchon=all&keywords=GeForce+FX+5200
They still have the problem of incompatibility with xserver:
The following packages have unmet dependencies:
xserver-xorg-video-nvidia-legacy-173xx : Depends: xorg-video-abi-10 but it is not installable or
xorg-video-abi-8 but it is not installable or
xorg-video-abi-6.0 but it is not installable
Depends: xserver-xorg-core (< 2:1.10.99) but 2:1.11.2.901-1 is to be installed
Cheers, Jonathan