The 4200 actually outperforms the 5200 in a lot of benchmarks when not using AA or FSAA, and performs the same as it when you turn those features on. But... in DX9 stuff, well, the 4200 doesn't do DX9 stuff.
Some more from a recent review here on AnandTech:
When looking at the actual products themselves, you can make a couple of generalizations about their performance. For starters, the GeForce FX 5600 Ultra performs much like a GeForce4 Ti 4200 in situations where no anti-aliasing or anisotropic filtering is used. Enabling either or both of those features allow the 5600 Ultra to significantly outperform the GeForce4 Ti 4200, mostly thanks to the NV31's superior memory controller, compression and AA engines.
The GeForce FX 5200 Ultra performs slightly above the level of a GeForce4 MX 460 in situations where no AA or aniso is enabled; but once again, enabling those features causes the 5200 Ultra to perform more like a GeForce4 Ti 4200 than a GeForce4 MX. The reason behind this is obviously not because of any compression algorithms (as there is no color/Z compression in the NV34 GPU), but rather because of the AA and anisotropic filtering engines, as well as a vastly improved memory controller when compared to the GeForce4 MX (not to mention the higher clock speeds).
It's certainly a toss up for you. I would get it, only because it can do things the 4200 can't (DX9) and with driver revisions it will certainly get better. Heck, I'm thinking of sending you my 4200 for that 5200.
