Music quality, best format for iPod?

I am an osx user but i also have xp this is my first time need to learn. I have a firewall up now trying to get it down My number 1# problem with limewire is the quality of most the music ic there a way to ajust it or do i have too work from my xp side and what is the best format for ipod, Im sure these are basic questions too all of you but I have no clue also how do I keep Itunes from removing songs if anybody wants too help thankyou
Tmack
mackbookpro osx 10.5 winxppro cable mdem fastest they offeer