Well of course the entire FX series sucks. I don't think anyone's disputing that! 😁 I have a FX 5900 Ultra running 500/450 and it gets its ass handed to it by a Radeon 9600 in Half Life 2 DX9 mode. I even tried Oblivion on it and its only barely playable at 640x480 with Medium settings. A 9700 Pro runs Oblivion probably 3x faster.
I think it was how there were three cards with the Ultra suffix that got me curious. 5200U, 5600U and 5800U. I think the 5200 Ultra performs like a GeForce 3 Ti 200 in DirectX 7/8 stuff. The 5600U like a GF4Ti 4200. And the 5800U, which they obviously clocked as high as possible, performs similar to a 9700 Pro as long as you keep DX9 games far away.
There are actually two different 5600U cards. They changed chip manufacturing after a bit and the later cards run 50 MHz faster (400 vs 350).
http://techreport.com/articles.x/5660
The FX series are actually quite nice cards for games before DX9. They have better and faster anisotropic filtering than the older GF cards (assuming you use drivers that don't cheat too much). They also have some interesting anti-aliasing modes but the Radeons have far superior MSAA (due to gamma correction and up to 6X vs 4X). And of course they have NVIDIA OpenGL quality, and some games also used NVIDIA specific extensions back then or were specifically developed on/for NV hardware (see Bioware).