It might help to disable PnP OS in your BIOS, if it's enabled... this option is pretty much stupid, I don't know why BIOSes even have them. I'd trust my BIOS over any OS to assign the right resources to stuff.
Or maybe "assign IRQ to VGA" should be on?
Otherwise, yeah, try "mem=nopentium" if you're using one of the original Athlons (not the XP or MP). Or actually, try it anyway, even if you do have an Athlon XP or MP. But on Intel processors, this won't solve the problem, because the Athlons do some "advanced speculative caching" that Intel processors don't do.