There are really only a few basic things to ask yourself:
- Will I be traveling with it?
- Apple, Intel, or AMD?
- Main use - home, business, or gaming?
- Preferred operating system?
So let's say you won't be traveling with it, you prefer Intel, you're a gamer, and you want Vista. We'd suggest a Dell XPS - you can get a high end cpu, dedicated graphics, tons of memory, a large SATA drive, a 17" widescreen, and Dell's excellent support.
Maybe you're into Macs, do graphics work, and travel - then we'd suggest the Macbook Air. You'll get a light laptop, high end graphics, widescreen, and the OS you're comfortable with.
In fact many sites today will ask you similar questions to guide you in the right direction. If you're unsure, pick up the phone and talk with someone at the company.
Best of luck in your searching!