Don't be surprised if you don't see much difference between the two cards. The FX 5200 should be a little better, but I don't believe you'll see a big difference. I'm basing this on peronal experience with both cards.
However, if your game requires DirectX 9, you should go with the FX 5200, which was the first nVidia card with DirectX 9. The Ti 4200 is DirectX 8.