Will this new crankset be compatible with my current setup?
 

Currently have an FSA Gossamer 36-46T with MegaExo BB and 12-25 cassette. I want to change the crank to an Ultegra 6600 39-53T and HollowTech II BB with the same cassette. I am pretty sure my current chain is gong to be too short so I was also going to pick up a 6600 107 link chain but not sure if that is long enough. Will the new crankset and BB work with my current RD and FD?

What front derailler do you have? There are a few "compact-specific" front deraillers out there but most bikes with compact doubles come with a standard double FD (which is exactly what your new crank is designed to work with).

You are correct that your chain will be too short with the new crankset (slight chance it will be ok depending on how it was sized but highly unlikely). Where are you finding a 107 link chain? Standard chains are 114-116 links.

As for the rear derailler, your new setup is nothing out of the ordinary and basically any rear derailler will be able to handle the amount of chain wrap your new setup requires.

How do I find out what kind of FD do I have? I found the 107 link chain on ebay for 20 bucks.

They are typically labeled on the outside of the cage. If you can't find any label or part number (FD-XXXX for Shimano), post your bike's model and year and I'm sure the info is available somewhere online.

Someone must have cut that chain down then or possibly removed it from an unused bike (if they are claiming it's new). If you haven't already purchased it, I'd suggest buying one of these instead: http://www.universalcycles.com/shopp...&category=2768