I would have Googled this question, but I wasn't sure what to search.
Ever since I got my rig in December, I've been using CCC to force everything graphical on my games (e.g. 8x AA, 16x AF, etc.) with my XFX 4890.
I was going through the InCrysis forum this morning looking for a particular post, and stumbled across a 5970-Crysis issue that could only be resolved by using CCC to force 8x AA, 16x AF, etc. and turning off the in-game AA.
This led me to wonder if using CCC 8x AA and Crysis AA (or any in-game AA/AAA/AF etc.) would drop my framerate, because the card is having to do the work twice, or if nothing additional is happening.
Both ideas sound pretty realistic, so I am a little confused.
All I know is that I have heard recommended many times not to have AA enabled in game and in CCC. Most times it probably does nothing, but I believe it can cause bugs or slowdowns/crashes. Also, if the game has an AA option, it is usually better to use it (more optimized).
Generally speaking forcing AA through the drivers will take a bigger chunk out of your performance than in game AA will. As said its not a good idea to try and have two different instances of the same thing running as it has the possibility of causing conflicts which would then cause bugs/slowdowns/crashes.
It is my understanding that forcing AA or any other 3D setting from the driver overrides the in-game setting. Therefore its not doing double-work if both are enabled, with the CCC setting taking precedence.
However, you will find that the native in-game AA is smoother in most cases than that which is forced from the driver in CCC. So setting to let the application decide is the best choice if it has any in-game AA settings.
It sounds like the 5970 might have some sort of bug where the in-game AA simply doesn't work, and only driver-forced will work. I wouldn't count on your 4890 having this same bug. I guess you'll never know till you try the settings. It won't harm anything to do so.