The Monitor determines the Max resolution (up to a point).
I am using this setting with a GF6600 (but I couldn't get this setting until I recently bought a new monitor), so your card should be capable of it, are you using XP?
Right Click on the Desktop
Uncheck Hide modes this monitor can not display
You should be able to try it.
Do not click OK if the display looks wrong when changed and it should revert back to the original settings.
Don'T know the procedure for Vista but it may be the same.