The latest-and-greatest NVIDIA PCI card appears to be the 6200, http://www.chumbo.com/Info.aspx?id=304966
, which is a NV4x series GPU core. I've never tested a 6200 PCI so I cannot comment directly on performance.
The FX 5700 performs better than any FX 5200/5300/5500. Even a FX 5700 LE will blow away a FX 5200/5300/5500. The 5200/5300/5500 all share the same less-capable NV34 core logic, while the 5700 is NV36. If you stumble across a FX 5600 PCI card (NV31), it ranks below the 5700 and above the 5200/5300/5500.
Ignore GF 4000 (NV18) or GF 2 (NV11) . Too old, too slow. It's a wonder that consumers still buy them, but the $5 GPU still has an OEM marketshare.
Despite the availability of a 6200 PCI, a faster GPU than your 5700 may not display better performance if you play with high texture detail and high resolution displays. The PCI bus really limits bandwidth.
My experimentation suggests that raw GPU horsepower has become irrelevant on the PCI bus. For example, my 5200 AGP doubles the Doom3 framerate of my 5200 PCI. This is bus restrictions, not GPU restrictions. Thus I can't expect faster data transfer from a PCI 5700 than a PCI 5200, but the PCI 5700 should offer better texture quality, anti-aliasing, filtering, etc.
You might think that any GF 6 generation GPU should
be faster than any FX (5x00) GPU, but this has not always been the case. The low-end of a new generation often offers more image quality but less horsepower than the high-end of the previous generation.