The GF4 ti 4200 is a good card and unless you really have the money to fork out for a ti 4600, the ti 4200 is your best bet. Unlike the GF4mx card which is just a Geforce 2 core with some Geforce 3 capabilities in it, the ti 4200 is a *true* GF 4 just with a lower clock speed than the ti 4600(and if you get a 64mb version, obviously 64mb less ram than the ti4600...they do make 128mb versions of the ti 4200 though I believe).
But yeah, I'd really recommend the ti 4200 as a budget card. That said, if you can hold out about a month longer, ATI's new Radeon cards are about to hit the market...while the price vs performance is definately not worth it in my opinion, the new ATI cards, since they will be the fastest on the block for the time being, will more than likely cause Nvidia to drop the price on the whole GeForce 4 line.
So, if you can hold out another month/month and a half, you'll probably see a nice price drop on the ti 4200, 4400, and 4600 cards making them an even better deal. But yeah, switching to one of those three should net you a very nice boost in gaming performance.