Pioneer is the brand that Apple usually ships with their machines. I'd suggest going to NewEgg.com and buying the Pioneer DVR-106. It has 4x DVD burning in both DVD-R and DVD+R formats, and it also supports DVD-RW and DVD+RW. Currently it's $121 with free shipping.
As far as installing it, there should be a detailed guide somewhere on XLR8yourmac.com, but good luck finding it. That site is very convoluted and difficult to navigate. Still, inside there should be an IDE cable running from the motherboard to the Combo drive. All you have to do is set the jumper on the drive, put the new drive in place, connect the power, and connect the IDE. The IDE cable has two connectors at the end, one for the main drive and the other for any new drive you may want to add. As for the jumper settings, one has to be Master and the other has to be Slave. Just check the Combo drive to see which it is set to, and set the Superdrive to the other.
Once installed, you can use the eject key to open one drive and option-eject to open the other.