RH7.2 & Radeon 7500 troubles

I'm running RH7.2 and I got a new toy -- a Radeon 7500. Realizing that just
plugging in the new board would cause the xserver I was using to choke, I
set the xserver up as plain-vanilla VGA before the switch. Choked anyway.
Re-installed RH7.2 (scratch -- reformatted the drive) and it choked while
trying to set up the X configuration. Did some poking around and found that
the 7500 *should* be supported by XFree86 4.2.0. OK. Downloaded 4.2.0,
reinstalled RH7.2, skipped the X-configuration, ran the Xinstall script and
vola! startx produced twm as the window manager. OK. Ran gdm or kdm to try
to get to the selection of a window manager. Login screen came up properly,
but any attempt to change the display manager to kde or gnome was ignored --
still comes up in twm. Now, during the Xinstall script I was asked a bunch
of questions about changing links (or not), the answers to most of these of
which I am totally clueless. I suspect the problem lies here. I tried both
extremes -- change nothing -- change everything -- still no success, or even
a hint of success. Any guidance appreciated.

Thanks, Ed.