Web designers use more highly specified monitors and computers but should make sure that their pages work for the majority of surfers at 800 x 600, 16-bits. Using type is more confusing. 12 Point type on a Mac is about 12 pixels high, but 12 point type on a PC is a third bigger - nearly 16 pixels high. The flow of the text columns will be considerably different from one machine or browser to another. The fact that pictures stay a constant size and type doesn't, is potentially the Web designers' greatest trap.
People often ask which is the best resolution to scan Web images.
There are a number of issues here. Desktop scanners are similar to LCD monitors in that they have a 'natural' resolution corresponding to one element per pixel. My Apple Flat Screen Display's natural resolution, for instance, is 1024 x 768. Neither 800 nor 640 divide evenly into 1024 and there is no such thing as a half pixel, so the image is fuzzy - same thing with a scanner. A scanner will produce the best results at its natural resolution, this is most apparent on images with regular patterns.