what kind of an fps increase might I expect? Small, big, medium?
Huge? :)
Seriously, Matrox is no good. I had one years ago, although can't remeber what G-model is was... but it couldn't do even multitexturing right! It had good picture quality though.
Even measly GF1 has T&L which means that GPU handles transforming and lightning calculations freeing CPU to do other things. This is a BIG advantage, even for Blitz3D apps which is only DX7. And upgrade to W2000PRO if you want to upgrade the OS - but anything is better than W95!