The Ti 4600 will clearly beat the FX 5600.
Reasons:
- FX 5600 (NV31) operates on a 4x1 rendering pipeline (4 pipes + 1 texture mapping unit per pipe), whereas the 4600 operates on 4x2
- FX 5600: 325/500 MHz core and memory frequencies, respectively. Ti 4600 = 325/650 MHz
- FX 5600: 8.0 GB/sec. max. theoretical bandwidth. Ti 4600: 10.4 GB/sec.
The FX 5600 Ultra can be competed with the Ti 4600, but the performance issue there is pretty much 50/50. One weakness of the Ti 4600 is the lack of many features introduced in DirectX 9. There aren't very many DX9 feature-enabled games out on the market, but expect more on the way. For now, if you intend to keep the card for 6-10 months, the Ti 4600 is a good idea.