I've built a custom 1680x1050 res at 60Hz which works fine with my Dell 2005FPW. There's a tar file here
with three files in it: the .vfs, which is the description of the format, the .sdb, which is the compiled format and should go in /usr/gfx/ucode/MGRAS/2RSS/ and a script call_vfc which runs VFC to turn the .vfs into the .sdb should you want to modify the format. You'd need vfc installed to do that.
Just stick the .sdb in with all the other .sdbs in the folder above and the format should appear in Display Properties.
You might want to check if that screen supports sync-on-green, which it needs to to run off an SGI. That said Dell makes no mention of that but mine works fine... your mileage may vary :-)