To reiterate what Paul said:
You can use the music you bought from the iTMS on up to 5 computers at any one time. You can deauthorize a computer at any time and use it on another computer without ANY problems.
The music you have already and any other music you rip yourself will NOT be restricted in ANY way. iTunes doesn't add copy protection to songs, it only honors the DRM in iTMS songs.
You can use any number of iPods with your iTunes. There are no limitations on that at all.
You can also copy all the music you've got on your PC in musicmatch to iTunes. A word of advice: let iTunes manage your music for you. It'll tag the songs properly (so you can easily find them on your iPod) and sort the folders in a very logical way.
Bottom line is that you shouldn't worry about any restrictions on your own music and there are just a few that your need to think about with iTMS music. HTH. And welcome to the Mac.
