Dygs,
No, the Geforce4 Ti is not worth it. You won't notice a difference in 10.2; I have tested, and there is no difference between the 4Ti and 4Mx. In future versions of OSX, with more intensive effects, you might notice a difference at the highest of resolutions, but such new "intensive" effects are a long way off. By the time OSX is able to take advantage of the greater processing power of the Ti, new and better cards will be out (and the Ti will be a fraction of what it costs today).
Indeed, in late November, nVidia will release its next-generation chip architecture. It will not be called the Geforce5, but will have an entirely new name (the Geforce brand will be retired). nVidia claims the new chip will deliver as much as a 50% improvement over the Geforce4 Ti4600, if I recall correctly from the latest interview. And the new nVidia card won't just increase frame rates, it will add support for all sorts of new lighting and other programmable / custom-defined effects.