I'm with Petermarrek. I got a decent laptop late last year for road trips, and with 4 gig ram it actually ran Photoshop faster than my older desktop with the same amount of memory. But the laptop has always been more of a pain to work with due to the smaller keyboard, lack of flexibility, video output limitations and internal disc space.
My desktop died last week, and I bought a new one. It's amazing how much prices have dropped even as capacity has increased; you now get a quad processor and 8 gig of ram (at least for a PC; Macs are still overpriced) for barely over $1,000 U.S. off the shelf. Photoshop, Bridge, and all the other image editing software I've come to depend on are far, far quicker on the desktop. Very good quality video cards and built-in high speed card readers are now pretty much standard.
So unless you're really cost constrained, you'll be much happier using a desktop as your main image editing tool.