Originally Posted by AndrewG NY 
I would be careful when shopping for any of these as so many of the posted tests are shot using crop-sensor cameras. While the Tamron 28-75/2.8 enjoys a good reputation on smaller sensors, it might not be ideal on FF digital. Personally I think the 24-105/4 IS is probably the safest and most practical choice, perhaps supplemented with a couple choice primes for when more speed or more compact size is wanted. If this lens is too expensive, then I would think the 5D is probably the wrong camera.
Canon also has a pretty decent-looking new bounceable compact (bring anywhere) hotshoe flash, the Speedlite 270EX--which though unfortunately I don't believe it functions as a wireless commander--is pretty much what the AF-200FG *should* have been.
For the price and size the 24-70 is surprisingly unsharp. I didn't realize that until recently but having seen several examples it appears to be consistently true. That is why I went with 24-105 for my Canon system. I agree with you that if you're not going to match "L" glass to the mkII you bought into the wrong system.
The only flash that Canon makes that can be used as wireless commander is the 580. The EX designation would appear to indicate that the new flash can be fired remotely from a 580 but I would be shocked if it could operate as a commander