You might consider getting a Blue Ray combo drive. If you ever play Blue Ray movie disks, they'll look great on the iMac 27 screen. You can get them from around $50 - $80 for an external drive from Newegg.com or other online dealers.
I just purchased this one from Samsung. It's cheap, it's all white which looks good with the iMac and whilst not the fastest, it does what I want it to do, install software and play DVD's (I don't own any blur ray discs). Also, like the SuperDrive, no external power source needed.