The agpgart patch works only against the stock 2.4.20 kernel, so your version is a modifed(redhat ??) version, and thats why it wont patch correctly. you video card has nothing to do with that.
About the sluggishness, maybe you could tweaking some bios settings.? On board video uses your system memory ( or shared memory as they call it), but u have more than enough memory. I'm not quite sure what the problem is.