Yep, throw lots of ram at it, that will speed things up a lot. Also, dont forget the video card. Everyone is jumping on the 3d performance video cards, but there is a lot to be said for 2d as well. Look at benchmarks for the 2d performace when shopping for video cards. It can be important in image editing.
Intel or AMD? I dont have a preference, they are both about the same, with AMD being a little cheaper for the amount of processor you get. The motherboard, however, is a factor. Get the highest memory bandwidth motherboard you can get. I think right now, Rambus (yuck) is actually leading the way in this area. In that case, your stuck with intel.
Also, a BIG issue with large images, is hard drive speed and interface. You want fast drives, as fast as you can afford. Seagate has some good drives at 10,000 rpm, but they cost a lot. 7,200 rpm drives will do if you cant afford the 10,000 rpm drives. Also be sure the drives use the latest interface standard, ATA 133. This will help with burst speeds, if the information is already in the hard drive's buffer, it will zip out to the motherboard a lot faster. If I had unlimited funds, I would go with a SCSI ultra wide hard drive interface, and put a few 10,000 rpm drives on them. If your thinking RAID will get you blazing fast hard drive speed cheaper, think again. Look at the real world benchmarks, it just doesn't speed up the average hard drive reads. You will get more speed boost from the rpm of the drive.