I would guess I'm not the average photographer, I shoot ALOT of pictures when I'm in photoG mode (which is to say when I'm with other photographers or by myself shooting.) I normally don't go anywhere without a pair of 16G (or larger) cards for each of my 3 cameras.
My recommendation would be Sandisk 16G cards. The manual recommends a Class 6 card, if you get a Sandisk card, you should be fine with a Class 6 card. Class 10 cards are expensive, the pair of 32G Sandisk Class 10 SDHC cards I have set me back $400. If you are shooting ALOT of video, then the larger the card the better. But for photos, I would get a couple 16G cards.
Are you shooting alot of video? The T1i and T2i cameras have a limit as to the amount of video they will take at once - max size of a video file can not exceed 4GB. In 1080p mode, it's like 12 minutes. If you shoot video at lower resolutions, not only do you extend this limit to 18 or 24 minutes, but you also reduce the speed requirements on your memory card (you could use a slower card.)
What ever you choose, make sure of a couple things:
- There are plenty of fake Sandisk cards out their, make sure to buy from a good source (adorama.com newegg.com bhphotovideo.com) and stay away from ebay.
- Buy one card, make sure it can do what you want for video. Then buy a couple more if you want.
- Always have a spare card in your bag. I can't tell you how many times I've left my memory card from my last shoot in my computer!