AO sucks up cycles like a Dyson in a flour factory. I only use it on older games when it comes to forcing it on.
Newer games either have a built in version of AO that works fairly well or it brings your system to its knees when forced.
FXAA costs almost nothing in performance, and can make some severe aliasing look decent, but it can be at the cost of crispness in text, especially HUDs. I prefer MSAA if the hit isn't too bad.