Canon recommends a UDMA compatible card. Of the four cards tested, only my old 2GB Ultra II was not a UDMA card. For video, Canon recommends a read/write speed of at least 8 MB/sec. The old Ultra II is listed at 15 MB/sec, the Extreme IV as 45 MB/sec and the "Extreme" (no suffix) is 60 MB/sec.
Rob Galbraith has researched, tested and published data for the performance of many memory cards. I wanted to see for myself. My normal practice is to shoot in RAW+JPEG mode with the JPEG at the smallest size. I set the 7D for ISO 400, manual exposure, manual focus, IS turned on. Each memory card was freshly formatted in the 7D. My subject was the XNote Timer that I'd used previous to estimate G9 shutter lag.
I simply pushed the shutter button and held it down until more than 30 shots had been fired. Of course, I never shoot this way but I wanted to see what would happen.
All the cards performed the same for the first eight shots. That is, no matter which one of the cards was used, the 7D cranked out eight shots in about 0.9 seconds. Apparently, these eight RAW+JPEG fill up the internal (buffer) memory of the 7D. After those first eight shots, shooting rate changes.
With the Ultra II, the 7D required 21.7 seconds to fire off 30 shots; that is, 0.9 seconds for the first eight shots then 20.8 seconds for an additional 22 shots.
The 60MB/sec Extreme(s) and Extreme IV all performed the same as far as I could tell. With any of those cards, the 7D required 7.7 seconds to get off 30 shots; that is, the first eight shots required 0.9 seconds and the next 22 shots required 6.8 seconds. The timing was not uniform between shots. Two shots would be roughly 0.15 seconds apart followed by roughly a half second delay and then another two shots 0.15 seconds apart, etc., etc.
In practice, I've had a few impatient delays with the Ultra II in the 7D but not with the Extreme IV. I can't speak to any quality or ruggedness differences between the Extreme IV and the Extreme (no suffix). I've never had a problem with a SanDisk card. The Extreme (no suffix) are less expensive than the Extreme IV so, for now, that's my card.