Granted, I don't do a lot of action shooting...but, with one caveat, with the Sandisk Extreme 60 MB/s cards I'm using, for all practical purposes, even shooting RAW, the buffer is unlimited. I mean, sure, if I just held down the button for forever, it'd eventually slow to a couple frames per second, but it's just not going to fill up in any reasonable type of burst shooting. And I hate to think of the editing nightmare to deal with that many photos!
That caveat is, of course, that I'm just referring to the CF card. As soon as the camera writes to the SD card, it turns to molasses in January.
So, if performance is a concern, I'd recommend either leaving the SD card out entirely or leaving it in but only in "auto-switch card" mode, and then just to keep from getting a "card full" error message. As soon as you fill up the CF card (or come close to filling it up, pace permitting), swap in your next CF card, and just use the SD for emergency overflow.