I am a little confused about DPI/PPI. I did read the other post and even followed some of the links but I still had a few questions.

I want to design a continent or multi-continent map that I would eventually have professionally printed. The print media has a maximum size of height 48" and width 96". I don't think I will be designing a map that is twice as wide as is long so essentially, I will likely finalize the map as a 48" square. I understand that I want 300 DPI for printing a quality reproduction. By my calculations, I would need a document 300 X 48 = 14,400 x 14,400 pixels. Creating such a file in Photoshop 7 is memory intensive and working with a file half as big is slow. Does this sound right? If so, how do designers and artists work with such huge files?

Thanks in advance for any insight, comments, etc...