Custom Resolutions - possible or not?

I'm currently using an 8800GTX and a Samsung 225BW and I wonder if it's possible to set up a custom res and make it available within a game? Specifically I'm thinking about EVE Online, which I play windowed and which won't let me use anything between 1280x1024 and 1600x900. To be honest I'm less than clear about how these things work, but it seems to me that it ought to be possible to find a working res between those two? I've attempted to use RivaTuner and nTune, both with no result - all I get is custom res failed (when I've tried for instance 1440x900 or something similar).

If anyone has a solution to this, or can confirm once and for that it's not possible, I'd be grateful.