Looks like you will need to install a newer version of Xfree.
The driver will also not work on Xfree 4.2.0 or 4.2.1. It seems that it will only function with the driver (which is included with 4.3.0) in Xfree 4.3.0
Give this page a look
http://www.xfree86.org/~dawes/845driver.html
It looks like you will need to patch the kernel or just compile a new kernel, something that would be beneficial anyway, from the 2.4.18 install of woody, or if you installed 2.2.x. It also seems you will need to CVS the new version of XFree, unless you can find a deb package for it. Or you can be adventorous and apt it with the sources found here
http://www.linuxcompatible.org/print17614.html