My 2 cents on resolutions:
1024x768 (ancient, AFAIK nothing is produced anymore with this native resolution)
1280x800 (mostly tablets and phones, probably on it's way out)
1280x720 (on its way out, but supporting it can give the people widescreen gaming on the smallest and weakest of devices)
1366x768 (very common in laptops, will probably disappear in a couple of years but might still be useful as an option on systems that have a weak GPU)
1440x900 (becoming less common but still a good option to have on systems with a weak GPU)
1600x900 (quite a standard these days on affordable systems, should definitely be supported)
1680x1050 (still common, since it adds a bit of height to the screen that's useful for office work, should definitely be supported)
1920x1080 (THE standard, nuff said)
1920x1200 (less common than 1680x1050 but you might support it for the same reasons)
2560x1440 (enthousiasts like it if you support this resolution, but it will probably be overtaken by higher resolutions before it has had the chance to become popular)
3840x2160 (support this for those people who will still be playing the game >5 years from now)