PS: This guide is for AMD GPU's, not sure about nVidia GPU's.
I find the best image quality to performance ratio is achieved when downsampling 2560x1440 with 4x MSAA to 1920x1080.
Example shots:
http://img269.imageshack.us/img269/7186/highres1.png
http://img96.imageshack.us/img96/4677/highres4.png
http://img845.imageshack.us/img845/2091/highres3.png
http://img225.imageshack.us/img225/2940/highres7.png
Even if you don't find it to be much of an improvement in these screenshots, it's extremely pleasing to look at in motion.
Various notes:
- This will obviously impact your performance
- Can easily eat up a lot of VRAM if you are experimenting with even higher resolutions and AA levels
- Screenshots with Fraps/MSI Afterburner will be in native res
- Fraps doesnt' seem to support screenshots larger than 2880x1600, use MSI Afterburner instead
- This is the only way to get super-sampling in OpenGL games (on AMD at least) as the driver doesn't let you force it