My card is a Radeon 9000 Pro. (R250)
As long as you have the source for your kernel, you shouldn't have had to recompile it.
Here's the steps I had to take to get it to install correctly in Slack 9.0. Slack 9.1 comes with the kernel source, so makes it a little easier.
1) I recompiled a generic kernel, not exact to my hardware, but without extraneous stuff. Tailor it to your system. If you don't know what something is, or you're not sure that you DON'T need it, leave it in. Most things won't conflict. A timesaving thing to do is to make sure only soundcard modules, and net modules that you need are compiled. during Make modules and make modules_install, these can take time if you have many extraneous modules
Notes on kernel compile:
Framebuffer support if you want it, should use vesa vga instead of the ATI radeon module. I've had trouble with kernel radeon support, black screens, etc
Don't forget to make sure agpgart is compiled as a module.
I rebooted to make sure new kernel worked, everything else worked, etc.
Made sure support was included for ELF and a.out formats, as well as the basic module loader.
2) rpm2tgz fglrx<whatever>.rpm
3) installpkg fglrx<whatever>.tgz
4) cd /lib/modules/fglrx/build_mod/
5) ./make.sh
6) cd ..
7) ./make_install.sh
Then, I just changed the driver line in my XF86Config file to fglrx, made sure default color depth was 24, and bing bang boom, 3d Acceleration.
I would do every step of your kernel compile as root, just to help alleviate some of these problems. I realize it's not quite necessary, but that's the way I've always done it.
I'll be checkin' back on this thread
Good luck! It's not as hard as it seems... And if you do an upgrade to 9.1 anytime soon, just know that it'll be *very* easy to add accel.
-Shade