Since the OP has a 7D, I can pass on my experience. I have 4 16gb Transcend 400x cards and they work well in the 7D (and in the 1D MK3). I shot 1/2 of 8 volleyball games in one day (all in RAW format). That took up three of the cards. That night, I shot the state football championship game. I filled the 4th card and switched over to a 16gb Kingston 133x. I immediately just about came to a complete halt. I could shoot a burst of a running play, then have to wait and wait for the buffer to clear. Finally, I removed the card and switched over to a Sandisk Extreme III 8gb card to finish the game.
My 133x cards are going along for the ride just in case of emergency, but I don't plan to use them again unless I use them in the MK3. There is a big difference in 10mp sensor images produced by the MK3 and 18mp sensor size images produced by the 7D.
Bottom line, get a fast card or three and don't look back.
Edit: Hollis is right, you should get a high speed card reader for the faster cards. I bought a Lexar 800 Firewire CF card reader for my PC and a Lexar card reader for the express card slot on my laptop.