I just found out about Sager a couple days ago after looking at Alienware... so I can't tell you anything about that as it relates to Sager notebooks.
However, my Dell Inspiron 5000e has a UXGA screen. There are two options for it, you can choose to stretch the image to fit the screen or leave it as is.
For example, if I am playing a game like WarCraft III, where I know I can't run it at 1600x1200, I choose not to stretch the screen. Even using 800x600, the image will stretch to fit the 15" screen, and it results in a blurry look to it. All non-1600x1200 resolutions look like that, the smaller it is, the worse it looks, I have found. So, choosing not to stretch will result in the same clarity you get at 1600x1200, but you will get a black "matte" around the picture, like you would put in a picture frame.
So, to actually answer your question... no, setting it at 800x600 and having it stretch does not make it look okay. It will still have a "blurry" look to it. I can't imagine any other UXGA screen being different, but this is just my experience with a 15" UXGA with an ATi Rage 128 16MB (M3).