The fastest cards I've seen (both CF and SD) are the Lexar x133 and Sandisk Ultra III which advertise 20MB per second sequential read and write speed. They advertise this as a MINIMUM but I haven't seen any reports suggesting they work quiker than this.
FireWire 400 is rated at 400 megaBITS per second (50 megaBYTES per second), so there is plenty of spare bandwidth/speed whatever you want to call it, using a Firewire 400 card reader.
Using a 256 MB/sec Expresscard 34 won't speed anything up until the media cards themselves can read and write at anything faster than 50MB/sec. To take Expresscard 34 to the max it would take a Lexar x1700 speed card - THAT would be fast, but then the cameras would have to speed up their writing too.
My maths may be wrong, and I'm not tech guru who understands much of this, but from what I can work out a firewire 400 card reader is currently the fastest way to transfer images to your computer from the fastest cards on the market.
Having said that I'd rather like it if Apple would build in a mega fast card reader into the MacBookPro so I know, no matter what card I end up using in the future, I'm transferring it the fastest possible way to my Mac.