That depends on what you mean by stretch...
1280x1024 = 1.25:1 (3.75:3)
1280x960 = 1.33:1 (4:3)
1280x720= 1.777:1 (16:9)
1366x768 = 1.777:1 (16:9)
The lower resolution will run faster, less pixels. But only up to the limit of the amount of data your CPU can deliver. So if you lower it and the FPS stops going up, then you can safely run at the higher resolution with no concern that you are losing any FPS.