It really does depend on what you want to do. Personally I got a couple Lynx Expresses to control some strings that I already own, that will move from the outline of the eaves of my house into the bushes. I then got smart string controllers to put up flex strips to outline my house. For years and years I have wanted individually controllable lights to be able to do chase sequences with single lights. This is the first controller that can do that feasibly, and cheaply. If you don't want that fine control, then the Expresses will do you fine.
I don't have room to put in a mega tree, but if I could, I would build one of those with Smart String Pixels. I love the spirals people were designing with strings, but wouldn't want the tree to only spiral, and would want the capability to go from bottom to top also. The sequencing would be harder, but it is so much more flexible in making it do different things.
As far as what you need. Everything will need a dongle of some sort. At the moment you can get the USB dongle from RJ at anytime. With each Dongle you can control either 512 channels of Lynx Expresses, or the other non-smart string controllers, like MR-16 or SSR-4. This would also include the Aether or Aether 2 flood light, which I believe will be available before this Christmas. The same Dongle can be programmed to control up to 4096 channels of the smart string controllers, which would be 1365 individual pixels. Each pixel has a red, green and blue on a single LED so needs 3 channels per pixel. Each controller can handle a string up to 128 pixels long. They plug into a power distributing hub that splits the signal from the dongle to each one, and supplies power for the controller and lights from a computer type power supply. The hub has 16 ports to plug into, but you are limited to the number of pixels per hub to the 1365. So if you do go with them, you will have to plan accordingly. There are also mini hubs available that can split one of the hub's ports to another 4, so you get 3 extra ports per mini hub. This could be used if you want a few controllers far away from the hub, and you would only have to run 1 data cable to them, plus a power cable.
The Hub also has a DMX output that you can use to control up to 512 channels of the older style controllers as I mentioned above. In your case you could have the first 128 be for Lynx Expresses, then start at channel 129 for Smart Strings if you want to use them. For each Hub you use, you would need a separate Dongle. There is a combiner that would combine up to 4 dongles to run on 1 cable to the first hub, and then you can daisy chain the hubs out in the yard. In the future RJ is working on a dongle that runs on ethernet so it could be controlled over the internet and would plug into your home router. This dongle outputs 16384 channels, so 1 of them would also control 4 of the hubs. And he has let out some tidbits of info of future upgrades, like not needing a show computer to control it and download the show directly to be run on the Ether-dongle. He has stated this will not be ready for this year.
Hopefully that is a good primer in what is available and what is needed to use the different controllers and don't be afraid to ask more questions here, everyone loves to help out.