monitor resolution question

So I went out to best buy and bought a Samsung 245BW.  I like it so far cept for the vertical viewing angle, which makes it a little dark at the top of the screen but that is inherent in all TN panels.  Good thing I can return it within 30 days if I don't like it.

Anyways, I figured I would test it out with some games and see how it looks.  Somehow I can run HL2 fine on 1920x1200 full AA and AF almost without a hitch on a X850XT.  Only thing is... say in some games I want to scale it down to 1680x1050 (like in Titan Quest) for testing purposes, why do the graphics not look as crisp?  I used to run a 21" Samsung and 1280x1024 looks 2x better than the new monitor on 1680x1050, which is still a widescreen res.  I don't understand how that can be.  Do I always need to run games at my native 1920x1200 resolution to get good results?  I can't do this when games get more demanding.  Is it just the fact that im using the X850XT?

Obviously I'm not looking too into it because my DS machine with 2x 8800's is coming in today but I will be concerned that a year from now or however long it takes when I cannot run a game at 1920x1200 and have to drop it down to 1680x1050. 

Can anyone provide some insight into this?