I had this very same problem about a year ago, and all anyone could tell me was "read the readme" was was part of the problem.
They don't seem to know what they are talking about or is an old readme they never updated. Here is my notes...
Working version, 2.4.5 and XFree86 4.2.0 (slackware packages, tgz)
From /usr/src/linux-version/ type make menuconfig
Go to processors and disable SMP.
As root, and where kernel driver and glx driver are located type
In /etc/rd.d/rd.modules add the following...
That should at least get you close. I don't know why agp won't work with 2.2.19, but I beat my head on it for about a month, and as last desperate act, I reinstalled with 2.4.5 and d/l xfree 4.2.0. Shotguning it like that seemed to do that trick.
Hope it helps. I need to make it work myself with Slackware 8.1 soon, so maybe I'll come up with another tidbit of info.