If you are running the full version of Photoshop or are running a RAW editor like ACR or Lightroom, watch your resource consumption on your computer as you do various tasks. You will see that the processor is almost never tapped out except when fully loading multiple images. However, the RAM saturates very rapidly. Buy 8GB RAM (or more if you work with 16-bit medium format / 1Ds Mk III images) for just about any computer, and you have just made yourself a "photography laptop" and at a fraction of the cost.
Jim
"Civilization began with distillation." - Mark Twain
"There is very little that good wine and dark chocolate won't fix." - Me
Canon 20D, Canon 20D-IR
Canon 17-40 f/4L, Canon 24-70 f/2.8L, Canon 100mm f/2.8 macro,
Canon 200mm f/2.8L II, Leica 50mm f/2 Summicron R, Minolta MC Rokkor 58mm f/1.2,
Contax Zeiss 50mm f/1.7 Planar T*
Canon 1.4x Extender II, 580 EX
Tons-o macro goodies
Gitzo 1340 tripod w/ multiple columns
Manfrotto 3275 Geared Head
Manfrotto 488RC2 Ball head
Manfrotto 685B Neotec Monopod
