Aspect ratios are weird because if they were normal, 16:9 would be the same as 4:3, but it is not.
Using your numbers:
1920x1080 is 16:9 (ok you didn't provide this one, but there is no monitor I am aware of that natively is 1920x1050)
1920x1050 is 64:35 (probably a 1920x1080 - 30 pixels for the taskbar)
1600x900 is 16:9.
1366x580 is 683:290
341x145 is 683:290.
1920x832 is 30:13
480x208 is 30:13
As long as you are playing on one of the screens it should work out nicely. If you happen to be using both screens, then you will need to make sure that the aspect ratio of the windows on the each of the screen are the same. The Window Layout Wizard should do this if you set the "Maintain Aspect Ratios" to true, and you choose a layout which spans both screens.
The only other thing that springs to mind that might cause an oddity is if the game does nto like the aspect ratio you are throwing at it or is it not picking up your settings properly. Sometimes you need to flick the game between "Fullscreen" and "Windowed Fullscreen" (or just Windowed) a couple of times in order for it to take. It depends on the game, but most of the time you want to end up on the "Windowed Fullscreen" (LOTRO will work quite happily in Windowed mode provided the resolution is the same as your main region).
You need to provide two things for me to help further.
1. a link to your profile.
viewtopic.php?f=18&t=132. After you have launched your game, open up the ISBoxer GUI (CTRL+SHIFT+ALT+G) and goto the Debug Tab, hit the Capture State button, and paste the results back here.