Make a profile for a game in Crimson (or Catalyst) and edit it like so:
AA mode - Override or Enhance app settings (Enhance if you wanna use it in conjunction with game's own AA)
AA samples or quality - 2x, 4x or 8x (higher number means better coverage/quality but slower poerformance)
AA method - You could leave at default or choose Adaptive AA. Adaptive is usually better than standard MSAA but has slower performance.
Supersampling AA (SSAA) is a huge performance drain so don't use that.
Bare in mind not all games can be forced AA through Crimson/Catalyst.