To be honest, you'd probably get better performance out of the card in there now for the most part. Given the other card is a MX, the 8X AGP and additional RAM would probably just about bring it up to the Ti4200's level of performance, maybe slightly less or slightly more, but nothing substantial enough to warrant swapping cards.
Still, if you wanted to, you might need to reinstall the video drivers for the new card, but it should be pretty painless by and large since they're both based on the same brand chipset.
I would like input before I start to upgrade my video card. I am running XP sp2, 2.0Ghz P4, 768 ram. I have a NVIDIA Geforce4 Ti 4200 4X AGP 64MB. I am going to replace it with a NVIDIA XFX G-Force MX 4000 AGP 8X 128MB DDR that I already have. Is it worth the trouble to put the 8X 128Mb card in place of the 4X 64Mb card? Is there any Known problems? Do I need to uninstall my current video drivers first?