I'm still using AGP Radeon 9600 + PCI-E Radeon X300 together (currently in zaphod mode) in my desktop machine (it wont handle anything newer or more powerfull), using latest Linux distributions (with KDE and it's compositor), without any big performance or missing features problems (no problems with games also). I've switched to open source radeon driver in around pre-last release of ATI's "binary blob" and drop support for R3xx-R5xx cards. I was really impressed how stable open source radeon was (compared to "binary blob") and that each feature that was there simply worked! Since then I was always happy with every version of open source radeon/r300/r300g driver. And support from Alex, Marek, Tom and other radeon driver developers was always great - very quick problems resolving (usually up to few hours after problem was reported there was a proper patch - well, my problems were usually in uncommon cases, when using multiseat with one or two cards, or video out to analog TVs, or some OpenGL features that "binary blob" didn't support on my cards).
Once more - BIG THANKS for all your efforts!