Yes, leaving a any port open matters. Most often hackers use compuers in large distributed attacks. So while it may not hurt you directly, by leaving a port open you're allowing hackers to attack major internet throroughfares.
Of more concern to you, hackers can use another computer as a bounce box making illegal activities seem to originate from you.
Thirdly, if a hacker decides to do so, they can waste your diskspace, cpu power, and bandwidth by installing a P2P server for instance.
Don't bother using linux if you don't already know it, instead try something like Sygate (Which is free and EASY) at »download.com.com/3000-2092-10184 ··· lst-0-5/