Problem with Aspect Ratio

[Sportsgamer]

I have a problem with my new monitor
Its aspect ratio is 16:10 the highest resolution is 1440x900
Not just NBA some other games I think are not show in the right ration even if I choosed the right resolution
Since I couldnt play at 1440x900 (too high for my computer; low framerate) There were no 16x10 resolution I did one myself with Pdubs guide I replaced the 800x600 (or sth like that) to a 16x10 resolution (1280x800)
Now when I play the game really looks ugly when I set the detail settings to 800x600 although I already changed the resolution. Whenever i take a screenshot with fraps, the picture has 1280x800 and the picture looks better than ingame!
Screenshots I´ll take later
Thanks