Canīt you set the screen size individually when in a game? controlling its W/H manually. You will get a black bar on the top and bottom but better that, than having a sqeezed/squashed screen to look at.
Honestly don't know how you controle the res manually for games to get the black bars. When I change the res in the game the image is always stretched out over the full screen. A note is that I don't like to play games in windowed mode either. Hmm... if I knew how to manually custom res in games I could play my old games without them being stretched out. 4:3 res'ed Mechs looks really weired in 16:10 res. XD
My screen is built is a 16:10 screen. I get a feeling that the 1680x1050 res I use, is sort of a custom res that either my screen or gpu supplies my computer with. Because the res's available to me in games and desktop is: 1680x1050 (16:10), 1600x900 (16:9), 1280x1024 (a weired 5:4), and then the rest are the normal 4:3's - 1280x960, 1152x864, 1024x768 and 800x600. So I only get one 16:10 res option to fit my screen. The "natural" lower res size for 1680x1050 would be 1280x800 I think.
Sounds to me like you have too much GPU for your CPU. You're not keeping your video system fed and that's limiting performance.
That could be a possability, because of the original idea of the computers build. I got this computer just over a year ago to replace my old agp computer which I bought back in spring 2003. The agp was not able to keep up with most games anymore.
Being on a limited budget I asked the shop to build one for me (my first custom built computer). The build of it was based on the main idea that it was going to be a very upgradable system, that would perform well in any game both before and after the need for upgrades. Last me at least 2+2 years befor having to think about getting a new computer. The first 2 with original build, and the extra 2 with upgrades. That is more or less what I got, and I am very happy with the build. It does perform very well. When it is not, is when I had decide to up the graphic levels a bit to much.

But yeah, as you said it could be that my cpu is not able to keep up with 2 gpu's, I don't know, I am not very knowledgeable in those exact things yet. It is just in the last year that I have started to understand and learn about the basic bit of computer parts and building, what things to use and all that. I am not at the clocker part yet. So I don't mind if you would give me a few pointers about the cpu and gpu relationship
The build:
Motherboard: MSI P6N SLI PLATINUM, 650I SLI ATX, Socket 775 (support for Dual-core and quad-core, so I could up to quad if I kept the mobo)
Processor: Intel Core 2 Duo E6550 2,33GHz S775 FSB1333 4MB Cache Boxad with fan.
Ram: Corsair XMS2 Xtreme 2048MB (2x1024) DDR2 XMS2-6400, 2x128Mx64,Heat Spr.
Graphics card: Gainward GeForce 8800GTS 320MB GDDR3, PCI-Express. Can support DirectX 10. (Obviously I got 2 of these now. The DX10 was so I could go over to Vista when the game torch is passed to them from XP)
Power: Antec Truepower Trio 650Watt.