beginning with cs3, adobe products have been intel native.
cs4 should be faster than 6.5 on an intel mac, particularly if you have enough ram to enable multiprocessing. cs4 has multiprocessing (also added with cs3), which allows ae to launch a render engine for each core that you have giving ae better use of multiple cores.
the catch is that each core needs ram to store data to process, up to 3.5gb per core (2gb per core is highly recommended)... so if you have a 4 core mac, you'd like to have 16gb+ of ram. with cs4, you can easily limit the number of cores that ae will use, and thus tailor the number of cores tht the amount of ram that you have, i just wanted you to know that speed has it's price, and while ae6.5 may have ran fine with 2-4gb total of ram, cs4 would like to have 2-4gb of ram per core to run as fast as possible.
even without multiprocessing enabled, cs4 should be a lot more stable than 6.5 due to the native intel code.