will i get faster cable modem speeds if i install a gigabit network adapter into my pc?
No, you will not get faster cable speeds w/that NIC card - but if you were running a home network (with say...20 PC's) and all sharing data back and forth all day - your shi* would fly w/the gig NIC card.
No, silly. Even 100MB card does not matter. 10MB is max on interface coming into your 'devices'