If you are not planning on using AA or AF, I would probably just stick with the card you have. There's really not much reason to upgrade yet, in advance of any DirectX 9 games. And I certainly don't see a reason to upgrade to a ti4600 (you could almost OC your 4200 that high if you wanted, at least in theory). Besides, the ti4200 is a good card ... you might want to just wait awhile for ATI to release their next card and then pick up a 9700 series when the prices drop.
That being said, I'm very happy my Radeon 9500 pro - it's a great card. Of course, I was upgrading from a GeForce2 🙂. But if I had a ti4200 right now, I'd wait until I could afford a 9700. Hopefully by then Doom3 will be out!