Firewire vs USB2 is unlikely to make a difference, unless its FW800. With normal FW (FW400), USB2 is actually faster (480mbits/s vs 400mbits/s), though for sustained transfer Firewire does slightly faster (but mp3 playing isn't sustained transfer, that would be copying multigigabyte files etc).
More likely just a RAM issue, iPhoto certainly grinds with not so much memory, and iTunes can as well, though not nearly so much.
Audion is popular and should be pretty lightweight, though if you _just_ want a player, quicktime should do actually, or even
VLC, which at leas has a playlist feature (and is anyway a great video player). Look
here for a whole range of options.
I would recommend the extra RAM, though, even f you have to wait a while to get it, with OS X, more than 512 makes a pretty big difference.