Thank you all 
spcomputing, I agree with you about the multilayer pcb in a pdf file, in which you can't select a layer anyway so what's the point! I tried to convert that pdf into a pcb file, but I can't find anything about it (tried "pstoedit" but no luck). But Chagrin is right, both 5v pins are connected together, and same for the GND pins.
Retrolefty I agree with dhenry, from what I understand the 12VDC go into the LM2576 which then distribute 5VDC to opto isolators, then to those 5V pins. I just wasn't sure about myself so I asked..I always ask when in doubt! And yes, I knew about converting the 16 analog to digital, we already discussed that in another topic of myself, I want to save pins because my project will be quite big in the future! Of course I will first try to connect some relays directly to the arduino, and once I get this working, only then I will try introduce the shift registers (buyed 5 of them so I'm ready to sacrifice some if things go bad :)).
About the opto isolator, I've read that they are model 817C, and require something like 3mA each. Again, I'm not sure, but I guess I will just have to try and see. I just hope I will not fry the arduino :P.
I think I'm now ready to buy this board and have lot of fun - or problems - with it!
Thanks again everyone, appreciated.