Please look into adding FXAA support. According to this article, it offers really good antialiasing with up to 40% less performance hit than traditional 4xAA.
http://www.hardocp.com/article/2011/07/18/nvidias_new_fxaa_antialiasing_technologyThe upsides are great antialiasing with minimal performance hit, and it can be enabled on DX9 hardware. It also helps with sub-pixle aliasing which could greatly reduce shimmering and sparkling in the game, if I read the explanation correctly.
It will also work on both nvidia and ATI cards.
The downside is that support for FXAA must be coded into the game.
