i've had 2 GTS's and 2 5500s. My bro who lives down the street has a Radeon
here's how they rank in my eyes (on a scale of 1-10, with 10 being the best). Tested on include MDK2, Q3, Q2, Q1, UT, Unreal, Deux Ex, 3dMark2000, NFS-PU, Evolva
2d quality
5500 - 10
Radeon - 10
GTS - 8 (up to 1024)
GTS - 6 (above 1024)
both the 5500 and Radeon look awesome. It's almost hysterical to see the difference @ 1600x1200 between the GTS and the 5500 or Radeon.
16-bit 3d
5500 - 10
GTS - 7.5
Radeon - 6 (ugh.....)
with a few driver tweeks (lodbias "-0.5", alpha blending "sharper", mipmap dithering "enabled", 3d Filter quality "high"

, 16-bit on a 5500 looks so much like 32-bit it's not even funny.
note-WITH DEFAULT driver settings, the 5500's 16-bit looks LAME. You gotta go in and adjust the drivers. Yet another reason why 99% of the website "reviewers" need a solid <thwap> upside their heads.
32-bit 3d
Radeon - 10
GTS - 9
5500 - 8
This one is tough. The Radeon's anisotropic looks a lot better than the GTS's. Thankfully, the postfilter is no longer enabled in 32-bit on the 5500. lodbias "-1.0" + mipmap dithering "enabled" ALMOST makes up for lack of trilinear. The GTS seems to take a pretty ehfty hit with anisotropic enabled, moreso than the RAdeon.
2xFSAA
5500 - 10
GTS - 5
Radeon - D'OH!!!!!
16-bit (with above tweeks) + 2xFSAA on the 5500 removes all banding and dithering associated with 16-bit.
I've taken screenshots of rocket smoke trails and water and lights and fog w/1024x768x16 w/2xFSAA. They look 32-bit. quite incredible. The GTS's 1.5 x 1.5 is worthless, IMHO. Someone said there was a game that the 1.5 x 1.5 looks decent. I never saw it.
Also note, switch the lodbias up slightly to "-0.75" or "-1.0" with FSAA @ 2x. This eliminates much of the "issues" that Ben just loves to harp on WRT RGSS and FSAA in general. Of course, he hasn't used the 5500 nearly enough, so he wouldn't know. heh...
The drivers my bro has doesn't have a "2x" type option on the Radeon.
4xFSAA
5500 - 10
Radeon - 7
GTS - 6
again, the lodbias slider makes all the difference here. It's not about "jaggies" with FSAA. It's about the obnoxious texture swimming and whatnot. the 5500's 2x removes it almost entirely, and the 4x...it's gone.
I think it is WAYYYYY extreme to say the GTS's image quality is poor. It's not. It's quite good in regular apps. It has a major advantage in a game like Evolva, which has Dot3. Of course, the Radeon does it a bit better, although I wonder if that's the 16-tap anisotropic of the Radeon vs. the 8-tap "anisotropic" of the GTS.
The GTS looks "bad" in 2d, "okay" in FSAA, "good" in 16-bit, and "excellent" in 32-bit
The Radeon looks "excellent" in 2d, "okay" in FSAA, "okay" in 16-bit and "superlative" (that's better than "excellent"

in 32-bit
The 5500 looks "excellent" in 2d, "superlative" in FSAA, "superlative" in 16-bit, and "very good" in 32-bit
There. That's my opinion. If you disagree, piss off. <g>