How can I?

I have an iMac DV SE 500Mhz and a NEC 19" monitor connected to it, in Mac OS 9 I can make the resolution bigger like 1280 x 1024 because the NEC supports it and I do this my using Switch Res but how can I do his in Mac OS X?
and uncheck the checkbox that says "Show Modes Recommended for Display." I had the same problem with my second monitor and this did the trick for me.

I did and it doesn't work because its going my the iMac monitor and not the NEC which does support other resolutions