If you use an hdtv for your display this may save the day...
I dont know if I can do a good job of explaining this because its from memory and I have no clue if anyone can benefit from this besides myself but here goes anyway.
I purchased an 8800 gtx for gaming and movies on my HDTV only to find that the powerstrip software for creating custom resolutions and timings would not work. The nvidia custom res app is very limited compared to powerstrip and did not allow me to make the timings and resolution needed for a 1:1 pixel mapping on my Samsung Hlp-5085w over VGA. My desired resolution is 1240x700 which fits the screen perfectly and is really just 1280x720 with no overscan. The problem with the Nvidia custom res app is that you cannot adjust horizontal scan rates so when I made the 1240x700 res it would send the wrong horizontal scan rate and kick my hdtv in to 800x600 while still trying to display 1240x700 lines. Of course this looked really screwed up... I had to settle for 1240x715 for a while which kept the scan rate high enough to keep my set in 720p. This caused a bit of overscan which was acceptable but annoying none the less.
I got curious about where the timings were stored in the windows registry and decided to run a registry monitor while adding a new custom res with the nvidia drivers. In doing this I found that a registry entry called CUST_MODE was added and of course the entire string was in hex. Using a hex to decimal converter (
http://www.statman.info/conversions/hexadecimal.html) I found that 715 (the horizontal section of my custom res was 2cb. Low and behold when I checked the reg key I found 02cb. There was plenty of other items in this cust_mode key and I assumed that the scan rates, front porch etc were the other items. Turned out I was right !!! Using the hex converter I found that 700 was 2bc. By changing both instances of 02cb(715) to o2cb(700) and leaving the rest alone I was shocked upon reboot to find that I had achieved 1240x700 with 1280x720 timings. I had a perfect 1:1 pixel mapping with no overscan. Resolution within a resolution achived.
I had to use a third party registry editor so that I could view the hex from the reg key in dword mode otherwise everything was out of order.
Create a custom res with the nvidia drivers so that the CUST_MODE key is created, search the registry for CUST_MODE. Even if you already have a 720p or 1080i setting in your display properties create one anyway. Make it close to the displays native res. Like 1280x718 for instance just to get the needed key created. Use the hex to decimal converter to find out what 1280 or 718 is in hex and then find out what your desired res is in hex and alter the appropriate keys. In theory this should work for anyone.
As always edit the windows registry AT YOUR OWN RISK and export the original key before you change a thing then of course. On a bad note it looks like this will have to be done each time you change vid drivers because it creates a new key with a different ID so you cant just install new drivers and import your old key...
Also tried with vista but no luck. Since the nvidia drivers for vista do not have the custom res tool there was no way to create the CUST_MODE key. I did try to add it manualy but no dice.... The only way I know to get custom res in vista with an 8800 series card is to install the driver, go to c:\nvidia and edit the nv4disp.inf file. In side of this file you will find a list of resolutions that are added when the driver is installed. Add the res you need and reinstall the driver from the setup.exe located in the same directory. You get no control over timings however so.... Umm good luck with that.
Hope this helps someone...