Do u think it´s possible using 1440x900 with a 320mb VGA?
According to your original post, you are currently working with a 19" LCD, which in other words means, you're running Oblivion at 1280x1024.
1280x1024 = 1.3MP and 1440x900 = 1.296MP (heck, lets round it off to 1.3, shall we).
So, unless they use some special chaos theory to render in 16:10 what they already render in 5:4, I seriously doubt that there will be much impact to your exisitng performance average. Note that resolution and VGA local Memory are not necessarily a 1to1 relation. Higher texture quality will have a much grater impact on local memory than a slight res increase, imho.
Of course, my whole argument is moot if you are running Oblivion at 800x600 and then allowing the Graphics Card drivers or Monitor to expand it to the panel's native resolution via interpolation...