It is not as easy as that. XBMC will give you alot of source material to stream, but it is finiciky and some links just do not work. The sports is the worst imo and never really works.
If you want to stream MLB on your apple tv it doesn't need to be JB. Buy the app photon and goto vipbox.tv and most every game will work, quality not the best.
If you have a JB ipad or iphone pm me and I will tell ya how to get HD MLB for no cost.
I, too, am looking in getting an apple tv as well and never owned one before. What are the advantages and the how to's in setting up an apple tv for a beginner? Is it similar to a jailbroken iphone?