IMO the safest way would be grabbing the source RPM, install it, patch the source tree then rebuild it. Using just the vanilla Xfree86 source is definitely not recommended, since Mandrake uses a number of patches on their XFree86. Obviosuly you'll need a compiler to do this so install GCC if you haven't already.
Grab the SRPM from any of Mandrake mirrors, here's one for you: ftp://mirrors.secsup.org/pub/linux/m...-23mdk.src.rpm
rpm -ivh XFree86-4.3-23mdk.src.rpm
The installed files are inside the /usr/src/RPM directory.
At this point switch to one of the virtual consoles then shutdown X (init 3),
also make sure you know where you downloaded the i810_drv patch you mentioned in your original post.
cd into /usr/src/RPM/SOURCES then extract XFree86-4.3.tar.bz2, which contains the vanilla XFree86 source tree.
tar xvjf Xfree86-4.3.tar.bz2
It will be extracted into a directory called 'xc'
Now cd into "/usr/src/RPM/SOURCES/xc/programs/Xserver/hw/xfree86/drivers/i810", which is where the i810 driver source is located, then apply the patch.
patch -p0 < /fullpath/to/the/patch
If it patches successfully you should see something like "Hunk #1 SUCCESS"
Now cd back into /usr/src/RPM/SOURCES, then tar the xc directory, replacing the original XFree86-4.3.tar.bz2
tar cvjf XFree86-4.3.tar.bz2 xc
Lastly rebuild the package using the SPEC file
rpmbuild -bb /usr/src/RPM/SPECS/XFree86.spec
The -bb option implies that a binary package will be built, in addition to the installation. If you don't want to keep a binary package you can use -bi, but I'd say its a good idea to have one, in case you need to reinstall. Note that the binary package will appear in /usr/src/RPM/RPMS. Also you may run into dependency problems when you invoke the rpmbuild command, if that is the case just install the packages it asks for from your Mandrake CDs. The build procedure will probably take more than an hour depending on the speed of your machine, since XFree86 takes a long time to compile.
If that didn't fix your problem(either the patch didnt work as intended or something went wrong), you might want to try the DRI cvs drivers: http://dri.sourceforge.net/
Good luck and Happy New Year!