Several months ago I had a problems with an FX5200 that would not allow me to set the resolution to 1440x900 under VISTA while using the DVI connector. I later discovered that switching to the VGA connector fixed the problem. Under Windows XP however, I had no such problems.
I would suggest that you do some experimenting, and if that does not work then you might have to install a different version of Windows.
Not sure about 98SE, but under XP it should be doable.
With the VGA connection it should just appear. If using DVI though you need to overcome the TMDS limit of the card/GPU. Since it's a mediocre quality TMDS it doesn't support resolutions higher than 1280x1024 out of the box. You need to force them, and it's tricky and a pain.
OK if you're willing to poke around in settings you may be able to add it yourself under custom timings (but read the instructions carefully and THINK first then act, this can screw things up) but it might not appear for you even after downloading coolbits etc;
From the change res window select the advanced button, then enter the resolution in custom display values for you H=1680, V=1050, Refresh=60, next change timing standard to CVT-RB reduced blanking. Remember it's progressive you want if it gives you any options.
You can add display resolutions and refresh rates using Powerstrip.
You can also add them using other techniques like regedit hacks, but those two are the easiest ways.
However if you can use the VGA connector, I recommend that route instead.