When you reconfigure (as root, of course), choose r128. Then restart X with a `/etc/init.d/xdm restart`.
Having r128 as your video card might make X come up, but unless you've magically specified everything else correctly (and the important parameters -- at least the ones I had trouble with -- are the horizontal and vertical refresh rates, and the screen size), it may be too small (which I never saw), distorted (which I saw a lot), or even turn white and freeze the entire computer (which I also saw a lot).
To prevent having to do a hard reboot (because when X crashes into the mysterious white screen of death, nothing -- not even the three-finger salute -- can send the machine the signals it needs to cleanly shut down), have your fingers ready over the CTRL+ALT+F1 keys to take you back to your console if the display is not perfect -- it might not crash if it's not, but if it does, you don't want to have to not only wait for it to check all its inodes (if you haven't yet tune2fs'd it to ext3 -- see Section 7.1), but perhaps screw up in the process of so many crashes.
If that's perfect, great! If not, read on ...