G5 M9590LL/A CRT resolution Help WTD

Hi, the graphic board on G5 dual core is NVIDIA GeForce 6600LE 128MB.
According to some site, Support for analog resolutions up to 2048 by
1536 pixels. However, when connected to my SONY 21 inch CRT monitor, the
max resolution is 1024x768@96/60MHz.

I used to use G4 Quick Silver-5 year old on the same monitor, which I
can go up to 1900 or something. Why my new G5 can't go even 1280? It's
pathetic. Is there anything I can do to get at least 1280? or is is the
hardware limitation-it's hard to believe?

Thanks in advance