I'll vouch for that! I just removed my 128 Pro and replaced it with a (fairly old) Radeon 7500, and the difference is obvious. While I couldn't get decent frame rates in Doom or Quake, I can now get 30fps. I couldn't watch iTunes TV shows before at all, now they are fairly well viewable, just a few small dropouts on some depending on the original video encoding (h264 is still a problem, MPEG4 is fine). Dashboard activation, Exposé, and CoverFlow just work, where before they were slow, kludgy, and almost unusable.

You can find older Radeons (like the 7500, 8500 series) for cheap (mine was $25) and they make a great difference. If you can afford a newer one, by all means buy one, but even an older one is worth your money.