I would consider using different and clearly marked cards for different types of photography.
When setting up HDR shots use one set of cards, when doing taking shots to be stitched together use another set and then one set for general shots.
I would definitely recommend AGAINST this. Although it sounds like it would make for easy organization, in practice you'll miss shots while you switch memory cards. Also, the photos on your various cards will not be in the order you took them anymore. (When I was going through my photos after my trip, it was really helpful that the photos were laid out chronologically. Churches/landmarks/statues/etc. that look similar when placed next to each other are readily identifiable when they're with other photos that are more distinctive.)
The mantra backup backup backup is worth saying lots.
+1. And it sounds like you'll be just fine here. Your 550D is the same resolution as my 7D, so I would guess your RAW files are 22-25MB each. If so, then your 120GB of SD cards will hold nearly 5000 photos. Then your iPod can be one full backup, and your external hard drive can be a second full backup, with plenty of space left over. If you run out of space on your SD cards, you can safely clear them for reuse, as you'll already have double backups. In fact, with your 120GB of SD cards, 160GB iPod, and 1 TB hard drive, you could safely store double copies of approximately 11,500 photos.