I'd actually encourage someone new to design (not sure if that is you or not) to start on a smaller display. With less real estate, it means less palettes can be open...which puts a premium on learning keyboard short cuts. When everything isn't "right there," you tend to learn them pretty fast.
If you're doing page layouts, then the ability to see an A4 spread at full-size is invaluable. Get a good quality 24" monitor and you'll work faster, instead of constantly scrolling and zooming in and out.