Thanks to everyone for the advice!!! I've decided to go with the 6600 12-27 cassette...which should work with my current 6600 RD...or so I was told by yet another LBS that I stopped in at over the weekend. So now I'm just shopping around for a good price on one. Oh...and I was told that I'd likely need to replace the chain (even though it only has 300 miles on it) since it probably wouldn't fit with the wider tooth range on the new cassette.