What CD-Rs the Xbox can read depends on which DVD drive it has. If your Xbox's disc tray has 2 extra holes in it, it's probably a Samsung drive, which is the best availible (reads almost all CD-R types, and is rated for 16x speed rather then 8x like the other drives). Personally, mine is the worst (a Thompson) so I can only get it to read my CD-RW discs, but you might have better luck.
Oh, and on a semi-related note, if you replace the IDE cable in the Xbox with a ATA 100/133 cable, you can greatly decrease load times and possibly music ripping speed. For some reason, it only ships with an ATA33 cable. People have reported a 30%-50% decrease in loading time by replacing the cable. However, you need a special type of screwdriver and to get at 2 of the screws you have to rip/remove stickers, which voids the warrenty. But the process is supposed to be very easy, once you get it open. I haven't done it yet though, I'm waiting until I trade in my Xbox to get a better one (I have a replacement plan thing and my Xbox is having trouble with some games, so basicly I get a free upgrade).