professional 3d apps like xsi need screens with a certain resolution, otherwise some buttons are cut off, yes there is a laptop mode that limits the application's height, but it's no fun.

one thing i would remember you of, we are approaching the decade of low-energy consuming, small, handy, styled devices, netbooks, pdas, handhelds. the eeepc and the acer one have a 1024x600 res, (casual) games that start on 800x600 will already be off screen, as you need space for the taskbar, the window decoration and the borders! tablet handhelds like the archos 7 series support only 800x600 natively, it can't run win games, but this might be important for web designers. dunno if it scales though, like the iPhone/iPodTouch.

so know your target customers and their devices. it would be unreasonable to design far cry 2 for 640x480, and also unreasonable to make a 2d chess for 1280x1024 only.