I agree on the plan to get one item at a time, but I will have to differ in the order of purchase, here is what I believe IMHO should be the order in this case,
Receiver > Mains > Center (and Side Surrounds) > Side Surrounds (if not purchased with Center) > Subwoofer > DVD Player upgrade > Rear Surrounds > Receiver Upgrade > Mains Upgrade + Dedicated Amp... you get the idea
The reason, to ensure all speakers are timber matched. Of course it goes without saying that all speaker with the exception of the subwoofer should be from the same manufacturer and of the same series if required.