That's interesting... it's set to "on" on both machines here, and I haven't seen any crashes yet (Via KT333, which also seems perfectly stable with AGP 4x; I can run RtCW, UT, Unreal (through the UT engine), UT2k3, Descent 3, even HL and CS through Wine). Same thing on my Intel 815 board.
Hmmm.... which chipset(s) have you seen crashes with? Which kernel version were you using (2.4.19 here, but I've been through 2.4.14 and 2.4.1[6-8] as well, no problems)? Kernel's agpgart, or nVidia's (I'm using the kernel's)?
Am I just lucky?