you dont need the etherdongle if you have a usb dongle already - usb dongle lets you do 1 universe of pixelnet (4096 channels) and etherdongle lets you do 4 universes.
After which ever dongle you choose, you will need either a 16 port active or passive hub. If you need to output DMX also, active is the way to go, passive is straight pixelnet. you will also need a SINGLE RAIL pc power supple with enough watts/amps to power what you are going to run thru the hub.
Then, you need pixelnet smart controllers (SSC) - one controller will run 1 light string - number of nodes depends on what type of light string you pick (128 max). The approved ones are listed in the wiki along with a link as to where to get them.
There is also diagrams in the wiki explaining how the pc > dongle > hub > controller setup can be done in several ways - suggest taking a look there as well.
Hope that guides you in the right diection....
Myk