Yes I'm pretty sure that's right. 640x400 = 32,000 bytes, so two pages would fit. The pixel clock would need to be 23.3MHz to get that resolution at 60Hz. I wonder if that's within a possible overclocking range? Maybe with a socketed board with faster CPU installed?

Beyond that, you're right, the changes would become so extensive that it would be much easier just to get a video card. Which might be the better plan anyway...