I know of two ways:
- use ENB mod (low performance because it renders at a multiple of the screen resolution and then uses downsampling to produce smoother edges)
or
- turn on MLAA in Catalyst Control (better performance but image quality might not be as good as with the method above).
Try both and decide for yourself. I used a combination of ENB (which adds a bunch of effects that can be turned off, though I didn't use AA) and AMD's MLAA when I played GTA IV.