Which mesa boards to choose for my mill?

Hi,
I have yet to find some kind of guide or walkthrough regarding what mesa cards will be suitable for which applications/scenarios. 
Probably because reading the boards' specifications is second nature to most people
I have an EMCO F1P CNC mill which I have been running for a little while with a HobbyCNC Pro driver board and open loop steppers. This mill definitely deserves better, so out goes the HobbyCNC and steppers, and in goes 400W AC servos from DMM with Dyn2 drivers.
I have figured out that the 7I76 will take care of all my needs with regards to I/O's, encoder (for the spindle), 0-10V (also for the spindle), but there are a few questions which remains unanswered:
- Which version of the 7I76 should I choose? 
- I see people are often pairing the 7I76 with another board called 5I25; what for? Should I?

The computer I am using is a very capable one, and it scored really well on the latency test. It has a parallel port, an ethernet port and a bunch of USB ports (if that's at all relevant).
So, which 7I76 board to go for (parallel vs ethernet, sinking vs sourcing outputs)? 

Thanks in advance

Hi Tommy. Thank you for your reply

So you would choose the ethernet version, and you would not go with the 5I25. Just for the purpose of learning a little, would you care to explain just a little bit about why you would do so ? Being a total newb, I just want to understand what the 5I25 would do/not do for me, and why I should choose the ethernet version over the DB25 version.
Is the setup the same across all versions ?

And by setup I mean in LinuxCNC, just to be clear

Okey, so I see why I should go with an ethernet card now, thanks.
I still don't understand why people are combining different cards though. Does it give functionality that the 7I76 don't have ?

Okey, fantastic,

So the whole "taking the load off of the computers' CPU" part is done by the 7I76 alone then? No performance gains from combining it with other cards?

What is there in the 5I25 card that makes so many people combine it with the 7I76?