The best thing is to give them static IPs. I have run into trouble with this before, because if you turn on the other computers before the one with the static IP, one of them might get the same IP as the static one is supposed to get.

Regarding the other thing, I really don't know why it wouldn't work to portforward. It is usually because of user error, but modem-->router-->PCs setups can be tricky at times.

And finally yes, your internet will load faster, not because of the static IP itself, but because of the other info you add as well saving the PC-->Router-->PC messages that would otherwise occur