That really isn't a good idea. 4.3 is infinatly better than 4.2. Why not look at setting up DRI (http://dri.sourceforge.net/
) with 4.3? That will allow you to use your cards hardware acceleration instead of the crappy vesa drivers.
The site says "Radeons up to R9200 are supported" and I'm sure your 8500 will be, since my 9000 is.
If you still insist on downgrading, you simply want to remove all the Xfree RPMS, thats is EVERYTHING that relates to them. And the install the RPMs of an older version. Some programs may not / probably wont work since they will have been compiled for 4.3.
Seriously, getting 4.3 to work properly is a much better option.