The R600 - R700 marketing was due to the fact that Shimano was not too sure if this compact thing was real or just a fad.... so they didn't set up compacts with the normal line designation.

The R700 was supposed to be between DA and Ultegra. I ride it and think it is more like Ultegra. The R600 weighs more and is more in line with the 105 group as mentioned. Is it worth the extra $100? Only you can answer that.

As for the whether or not you need a new BB....

You will need BB cups if you are not currently using a Shimano external BB. When I got my R700 from PBK they were running a special where they included the cups with the crank. Normally they do not come with the crank.

As for GVH selling an R700 for $155....I priced them out at the same price as PBK when I bought earlier this spring. They were sitting at $185. I have bought a crank from GVH before. I would highly recommend buying the R700 from them if that price is true.