I must disagree with Mylenium. I see a huge improvement from CS4 to CS6.
While using CS6 full-time last year, I took on some work for an agency that required me to use CS4 -- and it was painful to time warp back to 2008 like that.
CS4 is 32-bit, meaning the app is restricted to ~3.5 GB of RAM; CS5 and above are 64-bit, meaning they can use more RAM, which means you can get longer previews, and you can say good-bye to the "Unable to allocate space for a YxZ image buffer" error.
CS6's new global performance cache stores intermediate and final renders in memory and/or on disk, and it can be a big time-saver, especially while you're experimenting with a comp. Since Mylenium's post, there have been two point updates to Ae which have addressed the major bugs with the cache.
It's worth noting that the global performance cache is a set of features, and that it includes important changes in RAM caching. It is more than just the persistent disk cache, so you can see a performance advantage even if you don't use the disk cache.
You will need a 64-bit system to run CS6, and you will need to update all your plugins. As always, Ae will benefit from lots of RAM. New with CS6, Ae will also see a big benefit from a fast disk like an SSD for that disk cache.