CD-Rs are rated for write speed and possibly rewrite speed. I think 4x is fairly common. Honestly, though, it doesn't matter. CD burning software will test the media first (and you should do this at least once) and determine the maximum reliable writing speed.
Go for data CD-Rs since they are cheaper. Also the 80 minute CD-Rs offer more storage than the other ones but not all CD burning software supports them. Worst case though you'd use the 80 minute disk as a shorter one.
Finally, if you get into burning audio CDs, you may find that you are able to getaway with using the cheaper data CD-Rs.