Actually,
the FX5600U rev. 1 is marginally slower than the Ti4200. The FX5600 is still an improvement over the GF3Ti200, however, you will likely get a better deal with a Ti4200 (higher frame rate, lower cost). The scene changes when you enable AA/AF however, as the Ti4200 is much slower at processing AA/AF than the FX5600U. Without AA/AF to slow the GF4 Ti4200 down, the FX5600U is hardly able to keep pace.
If you got a really good deal for it, you'll be happy with the FX5600U as an upgrade from the GF3Ti200.
Let me know how it works out, I have a GF3Ti200 that I'm looking to upgrade, and have been eyeing either looking for a Ti4200 on the FS/FT forums, or picking up an FX5700 (which performs noticeably faster than the FX5600 rev. 1).