good points britboy.
also consider non apple promotions. many resellers offer to throw in a printer, digital camera, ram, or software for "free" when you purchase a mac from them. i think printers and ram are pretty standard offers from resellers, the others happen only ocassionally. shipping costs and times should also weigh in on your decision.
some points that may not be valid with you. remember education discounts if you are associated with education on any level (or know someone who could purchase for you who is). also employee discounts are often substancial (got a friend at apple? ask them about it).
its hard to tell someone where the "best" place to buy is since it is a highly individual process, but if you keep in mind britboys comments and a few of mine you might find yourself a good price.