1. It never hurts to have an extra battery. True, most DSLR batteries will last the average shooter all day, but it never hurts to have an extra.
2. Any speed will work -- I would go with Sandisk Ultra if you value economy, or Sandisk Extreme if you value maximum speed. I don't think you would be able to tell the difference in practice, really. Off-brand SD cards are ok, but Sandisk has a great warranty and is the most used among the photographers that I know (including myself).
3. A specialty store is probably best. Best Buy and other mass-market places will probably have just LowePro or Tamrac bags. Nothing wrong with them -- I have some of both, and they have served me for many years. I just outgrew them. Plus, they scream "camera bag" which can bring unwanted attention / theft. But they are perfectly good bags, especially for the price, and will do a good job protecting your gear. All the bags that I'm going to list below are significantly more expensive.
First decision to make on bags is what kind of bag to get. A waist bag (unlikely unless you want to look like a pro and/or a huge nerd), shoulder bag, a backpack, or a sling (like a backpack with one strap, that you can slide forward for easy access). These all have their ups and downs.
Some other bag options to look for: Crumpler X Million Dollar Home, where X is 1-8. These are shoulder bags that look like your average messenger bag; stylish and colorful. The 5, 6, and 7 MDH's are probably the most common. It depends on the size of the DSLR and how many lenses you have. I can fit my whole kit (see sig) into a 7MDH; the 5MDH is more suited to a smaller body (such as your XSi) and 2-3 smaller lenses. So the 5MDH would probably suit the gear that you are purchasing right now; 6MDH if you want to grow a bit. Although these bags are expensive ($80-$100 range) most people find they are stylish, practical, and very good at protecting your gear.
Think Tank. (order direct from thinktankphoto.com) These are more "professional" looking (solid black) and you can order ones that look like plain laptop bags/briefcases, etc. They are *very* well made and very protective of your gear. Again, they are pretty expensive (looking over $100 for most TT bags).
Domke. This is the major competitor to Crumpler in the shoulder bag area. They look more subdued but they are very high quality.
Kata. I have one of their backpacks, and it is extremely solid. It is the R-103 which has an integral laptop sleeve. It is definitely expensive (retail over $200) and I pretty much only use it for travelling. Kata makes other kinds of bags too.
4. Tripod. This can be very handy, but the cheaper ones can be pretty crappy quality (and many that you will find at Best Buy, etc. are really designed for video cameras, not photo cameras). The one cheap one that I can recommend is the Slik Mini V.
http://www.amazon.com/Slik-Mini-Tabl...dp/B000ASTKWQ/
It is $30, and it is small enough that it's about the size of my biggest lens when folded. It is great on a tabletop or on the ground. It's the only tripod that I travel with when I have to fly. It is extremely solid for the price, and can hold a pretty hefty amount of weight without slipping or tipping over. No, it can't completely replace a full-sized tripod, but it can cover 70% of your tripod needs in a tiny package that you could carry in a large coat pocket if necessary.
5. Lens cloth. Just a cheap microfiber lens cloth to wipe off your lenses with when they get dirty.
6. Card reader. Of course, your camera can just plug right into your computer with its USB cable.... but that requires your battery to be charged, and often it's slower than using a dedicated reader. I would pick one up if you don't have one already.
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