The DDR will stomp the MX silly
If you want something non-Radeon as a good secondary gaming card, might I suggestion a Kyro2? They don't cost that much more than a GF2MX, and in most cases will perform slightly better, but in some circumstances the Kyro2 can hang with a GF2Pro. And it has nicer 2D than the MX.
If you want something for a linux box I'd try to get a Matrox G400, the drivers are built into XFree, the 2D is top of the line, and 3D speed is around a TNT2, which is probably sufficent for a Linux box.
nVidia does have good Linux drivers, but I've heard they are a pain to install, the 2D quality is a bit lacking, but the 3D speed is better. Not sure if that matters to you on a Linux box or not.