If anyone is curious I did some research before going the SD card route. I actually looked into this when I set up a Win98 PC with an SD adapter.
One common thing to worry about with SD cards is repeated writes wrecking the storage chip, as NAND storage has a limited number of writes before it's dead. I believe this to be a valid concern but depending on your application it's not as big a deal as you'd think. All SD cards from reputable manufacturers (Sandisk/Toshiba, Lexar, Transcend, etc) come with wear leveling algorithms in the integrated controllers, which means it doesn't just write to the same cell over and over again if the same file is written more than once -- it writes to a different cell. This is effective unless your SD card is nearly stuffed full and you have data writing continuously.
For Xbox, most writes are savedata (small) and game cache (larger but not too bad). In this scenario, with a 200GB SD card at under 75% capacity (after filling with dvd2xbox game backups) you should have plenty of headroom to allow for cache partition writes without needing to worry for many, many years.
If you're very concerned about this 1) make backups and 2) there are high endurance SD cards from Sandisk, Lexar, and Transcend which are marketed towards continuous write scenarios like car dashcams and are rated for many many hours of writes, and they're not much more expensive than regular cards (just lower capacity ceilings), so buy those if you're paranoid.
