I'm going to agree with most of the other comments. If you already have a Ti4200, you're really not going to find a faster card for ~$100. At that price range, you are looking at Radeon 9600p-level cards, which do have DX9 support, but aren't remarkably faster than your current card. And I don't personally believe that $100 is a good price to pay solely for DX9 capabilities.
You best bet for low-cost performance increase is to either try to get a soft-moddable 9800SE (risky) or a used 9800p/9700p. That's for ATi cards. I don't normally follow nVidia's cards, so someone else can clue you in there.
I'd personally stick it out for a few months more, and hope the previous gen of high-end cards (9800/5950) drop to ~$140 or so. Might happen.