Pekka wrote:
I agree with mrchips. If I did not play games and make music with my PC, I would get a good laptop with a capable display adapter as the only PC.
I don't think I agree with the "only PC" bit. 
For a long time, I had a laptop as my only PC, and I thought it was a good setup. Some months ago I bit the bullet and bought a nice desktop system at a swap meet (so it worked out cheap, and I got to pick and choose what I wanted).
It is an Athlon XP1600+ (1.4 GHz), 768 MB RAM, 2 hard drives (40GB and 80GB), 20x speed CD burner and a 16x DVD drive. Coupled with a nice monitor, it was so much better for editing photos (and doing anything else for that matter) than my laptop, that I upgraded my laptop now! (I now have a Pentium 4 1.6 GHz with 256MB RAM, 30 GB hard drive, internal 8x speed CD burner). I am about to upgrade my laptop RAM to 512MB, and then I'll be happy! 
BUT, I still prefer to use my desktop for any critical photo work. The laptop is good enough to check your photos out in the field, but I find the display is not quite good enough to do serious editing work.
Sure, you could have a good, fast laptop, with an external monitor and stuff, but then desktop systems are really cheap now, and much more easily upgraded than laptops. I purchased this last laptop with the knowledge that it will be good enough for my D30 photos, and possibly quite a bit slower, but still good enough for my next digital camera (I'm hoping that the 1D replacement will be the one!
). So the laptop will "do" in a pinch and in the field, but I can easily upgrade the desktop in the future, so working in Photoshop will be fast and enjoyable.
Plus, plugging the external monitor into a laptop is a pain... 