It's probably the monitor, as I have a 19" CRT (On a 6600 AGP) as well and it won't go any higher than 1600x1200. Seriously though, have you tried just going to the monitor manufacturer's website and looking up your model?
Ok go to your advance button. Now go to the Monitor tab. You will see a check box that will hide all the resolutions the monitor can't support. Then you will be able to see what that monitor is capable of.
I forgot about those settings. ViewSonic rates my monitor's max at 1792x1344, but the tab allows me to take it up to 2048x1536. To the OP: I wouldn't advise running it at any unsupported resolution due to the risk of damaging the monitor; after all, they're hidden for a reason...although you did get the monitor for free, so it's not like you'd lose much if you tried. :twisted: