FCP 7 and Compressor 3.5 make simple iDVD like Blu-ray video discs. As others mentioned. DVD Studio Pro can't author.
If you're looking for external burner I believe Buffalo 8x got the best review from Macworld because it was fast and USB worked reliably with Toast at almost the same speed as firewire. I'd like to see the 12x reviews complete with burn times with specific good quality media.
If you have multiple computers, an external drive might make more sense. Internal you have to make sure you have the correct cabling for your specific MacPro as I believe there have been changes.
My main concern is data backup though. My corporate clients still ask for DVD even though I only shoot HD. I also give them HD files versions (even YouTube and Vimeo allow for 1080p now).