Well, upgrading your OS will not show a significant (if any at all) increase in performance in the form of FPS.
I would reccomend you say away from new ATI cards because take it from me, I've had nothing but problems with them. It took me forever to make mine stable.
I'd suggest you get a GeForce 3 at minimum if you are looking for a decent upgrade. You can usually get good prices herE:
http://www.newegg.com
they have some pretty off the wall prices (cheap *blingBling*). I'd stay away from all GF4MX cards because they aren't that great, and if I can recall the GF4 Ti4200 uses the GF4MX440 chip basically so there is little difference in performance between the two. For futerh information I suggest you look at some reviews,
http://www.amdmb.com
http://www.hardocp.com
-www.rage3d.com (for ATI scoop)
Run a google for hardware reviews.