There are only a few games out at the moment that realy push a GeForce4, but they tend to be the ones that are designed around the newer engines. Unreal 's new engine is used in UT2003 and Unreal2, and a modified version is used in Splinter cell and Rainbow6 Raven Shield, for example. All these can look pretty good with a GeForce4) but you probably wouldn't be able to turn everything up to the max unless you've got a 128Mb Ti4200 (to overclock) or higher. GeForce4 MX is one to be avoided unless you really want to spend as little as possible and you don't intend to play games like Doom3, Quake4, Unreal2, Half Life2, etc...with full graphics effects). Many would say that a top end GeForce3 is actually better then a GeForce4 MX.
Unreal2 was shipped with some of the graphics effects removed because no card existed that could exploit them. As I understand it, these are to emerge now that GeForceFX and top end ATI Radeons have pushed the hardware envelope further.
I have an Abit Siluro 128Mb GeForce4 Ti4600...the standard Ti4600 chipset of course, but Abit slapped a superb heatsink and fan on it that covers all the hot bits. It originally shipped at around £200, but then got rave reviews and was pushed up to £280!! I will be upgrading to either an nVidea GeForceFX Ultra 5800 or whatever ATI brings out to beat it around September, when Half Life 2 is due for release. I will probably have to spend a fortune doing this, and my current card is probably good enough to mean I won't need to...but, hey...I want to see absolutely everything in that game 8)
If your current card is running all the games that you like to play, and running them well, then you don't need to consider a change, but if you want to start playing some of the newer games, you could be struggling soon. If you aren't having problems turning all effects to the max then check the game engines that you are playing, even some recent games are still based on modified Quake 3 engines (or of same generation) and most average cards can run that quite easily. Perhaps the best thing to do is check your 3DMark score...I'll be surprised if it runs all of the tests (probably won't be able to handle the advanced pixel shaders)...but if it does, then stick with it until current GeForceFX's and latest ATI's start to drop in price.
If you tell me what you would be willing to spend on a new card, i'll give you some idea of a 'best bang for you buck' buy.