You could get an external CD writer or even external DVD/CD writer. But on the other hand if you want it just for backing stuff up maybe an external hard drive would be more useful to you and less waistfull than buying lots of CDs or DVDs. They're great for when you want to reinstall your operating system (and trust me the move to OS X is so very worth it). For either I have always found Lacie and iomega products to be very mac friendly and worth their money. There's loads of others out there though. If you have firewire ports I'd definately get a firewire device whether its a writer or drive.
I have one other suggestion for you which may be cheaper is to get a zip drive and disks. At least then when you back up your photos you can go back to them, edit them and resave them unlike with CDs. I'm sure you could even pick up a second hand one very cheaply.
Hope this helps.
