I find that each and every purchase/build just stacks on top of the previous. Going from partial mashes on the kitchen stove to AG in turkey friers was huge. I have my new 80qt pots and BG-14 banjo burners... huge... grain mill, immersion chiller, pumps, etc... I'm planning to pick up a 15 cu ft chest freezer tomorrow (for only $50 woohoo!) and in turn begin building my keg supply. I've had the supplier lined up for months, just waiting for the freezer before starting to buy.
I see it all as a natural progression. Generally speaking you're not going to have a 6 tap keezer if you're only doing some partial mashes on the kitchen stove, there's no need for the grain mill if you're only doing extract, not much of a need for a chiller if you're doing 2-3 gallon boils and can chill it pretty quick with ice in the sink (compared to 5-10+ gallon boils), using a pump for only 2-3 gallons isn't really needed, etc...
If I had to pick a certain thing I would pick the group buys on the forum. Since I can purchase a ton of grain (maybe only a 1/4 ton at a time) for prices waay cheaper than I would pay at the local LHBS it's saving me plenty of extra money to invest into all this other wonderful whatnot. I figure I am saving at least 50% if not more, so that's a few hundred saved (or re-invested).