6520G means you have a Llano. You also said "dual"; if it's really discrete+integrated GPUs then make sure you're running the drivers from the manufacturer of your system. Stock drivers may not work well on dual systems.
This is one issue with ATI GPUs and old OpenGL games.
http://compgroups.net/comp.sys.ibm.pc.g ... i-c/417522I used the method of putting atioglxx.dll from Catalyst 10.4 in the same directory with jamp.exe. I haven't seen the renaming the executable to quake3.exe mentioned before but you can try it.
As always, monitor temps etc... AMD Overdrive should do this.
If you are under Vista/7 you are running as administrator right?
How well do other games run? Particularly any OpenGL games?
Since you mention in crashes on server join, it could be a corrupted install, CD-check issues (CD/mini iso/Steam), map version mismatch, not having the 1.01 patch, mod/pk3 conflicts, or network issues.
Try a fresh install with nothing but the 1.01 patch, and join a basejka server (no mods).