nforce4max, dude.. You're right. It really seems that the reason why I can't get a higher resolution is because of my monitor's limitation. I just downloaded and installed the Nvidia Control panel which gave me a lot of options for the display. I was able to get options higher than that native resolution and any higher than 1366x768 looks weird.
I was probably able to select a higher res on the GTX 260 due to the fact that that GPU is actually deffective. Though it's working, it's not working properly (i.e., I don' have any option for a widescreen resolution, and Nvidia control panel can't detect the GPU itself).
Thanks again dude and thanks to spentshells too for making me feel better about my GPU. HAHA!