I'd guess its your monitor .inf not allowing higher resolutions.
I have a GeForce2 Ultra on a 21" IBM P260 (Sony Trinitron) & my resolutions stop at 1600x1200x85hz too.
When the specs clearly state my monitor will go higher.
And I
definately have the right .inf installed.
It never really bothered me though, 1600x1200 is more than enough.
Maybe you can try HZTOOL & check the boxes for higher resolutions & see if they become available.
HZTOOL