I have a Geckodrive G540 and I have just finished my control box. It looks like I will have to redo it if I use UC300ETH-5LPT combination but not if I use UC100. I've seen that UC300EHT-5LPT gives you more inputs and outputs plus analog inputs and outputs and you can connected it direct to a PC or via a router or switch. For only $44 extra its worth the money. What I cannot find is if it works only with its own network or you can connected to your home network so you can control it from any PC.
You should be able to plug the G540 directly into a UC300ETH with a ribbon cable. (Port 2 or port 3 on the UC300ETH.)
I believe that you can run the UC300ETH on a network, but it's more complicated, and you need to know what you are doing. I recommend running it direct, with it's own network connection. I use a $15 PCI network card to connect it.
And I agree that it's well worth the extra money over the UC100, as it gives you a lot more options for more features.
Since the UC300ETH will work with both Mach3 and Mach4, as well as UCCNC from CNC Drive, I'd recommend trying all 3 to see which you like best (and which works best for you).