This applies to both the 6000 series and the 5000 series. WARNING: Please don't try these instructions on a 7000 series, you can only use catalyst 11.12 and higher for the 7000 series, You'll likely experience issues if you try 11.6 as suggested in this guide.
1. Uninstall your current Catalyst drivers.
2. For the 5000 and 6000 series, download version 11.6 of catalyst, the newer catalysts seem to be degrading performance, at least in minecraft (Confirmed on my 6870, my friends 6670, and on my old 6570) Catalyst 11.6 32-bit Catalyst 11.6 64-bit
3. After finishing the install, REBOOT, If you don't, you'll still experience a lot of lag in minecraft until you do.
4. Adjust your catalyst control center settings to the following after rebooting:
5. Apply and launch Minecraft
It's likely that your Minecraft will run a lot smoother with the older drivers, I recommend that you experiment with your Catalyst settings after this to find settings that suit your card and Minecraft graphics setup.