Shimano America Ultegra SL FC-6604-G Crankset - Triple - 2008

The Spacing is Slightly Narrower causing a 9spd chain to drag on the outer chainrings more often and the hole system to be more touchy. You can try a 10spd chain this will help with the dragging but will shift worse so then you will need a 10spd Frt Der this will shift just fine with your shifter but will be narrower to handle the chain and spacing. Do to the fact that their are many variables involved with this conversion I would trust this to you local bike Shop.

I'm already running a 12-27 so I wouldn't gain much going to 28. Also, wouldn't I have to change the rear derailleur and chain?

Your Shifters are 10spd and Shimano has never offered a 10spd Ultegra Shifter that will do both so yours should be Double Specific So you need Shifters, Definitely your Front Der, a Short cage Rear Der will not take up the extra chain so you need a Long Cage triple Rear Der too and You might as well do your chain sometimes you need an extra link or 2. Have you thought of trying Shimano's new 10spd Ultegra Cass 11-28 (Just Released).

Another alternative is the Ultegra Junior Development cassette. The 16-27 cassette doesn't have a serious granny gear but does have a better range of gearing for controlling climbing effort assuming you are not hammering the descents.

http://www.backcountry.com/store/SHI0082/Shimano-America-Ultegra-10-Speed-Cassette-Junior-Developement.html?

If you run a 9 speed rear cassette, I agree with Christian in regards to opting for a mountain rear cassette and derailleur. Way more temporary, way more affordable.

If the shifters are newer shimano then the front shifter is compatible with a triple crank ($350). You would need a new front ($80) and rear ($125) derailluers as well as a chain ($30) and the bottom bracket ($50) you mentioned for total of $635.
A cheaper option would be a 12-27 cassette, assuming that is bigger than the one you have. 27 teeth (the size of the largest cog) is the most the short cage derailleurs can handle. You could go with a larger cassette (like a mtn bike cassette) but you would have to buy the long cage rear derailleur which has a 37 tooth capacity.
Good luck with your ride and have fun.

Yes The Ultegra would be an upgrade from any crank that is 3 to 4 years old. Your derailleurs should work just fine. depending on the current gearing up front. You might have to tweak the F Der.a bit.

The front crank set is not dependent on what the rear cassette size is. The only components that are dependent on 9 or 10 are the shifters, rear derailer, chain, and chain length. If the largest chainring is larger than your current size (count the teeth, the largest ring on this one is 52 teeth), you will need to buy a new chain to lengthen it. Do not try to add links to your current chain to lengthen it. The more and more times you cut a chain, the weaker it gets.

As long as you have a 10-speed system, you'll have no problems at all. Seriously consider the compact double though, it weighs less and gives you most of the gears you probably need:http://www.backcountry.com/store/SHI0066/Shimano-America-Ultegra-SL-Crankset-Compact.html

Since you're stuck on a 9 speed, the easy answer is no doubles for you (unless you're okay with a non-DuraAce crankset).Currently, all Shimano makes in a 9-speed compact double is a Sora crankset, which you can pick up from your LBS for ~$65. It uses Hollowtech II, so you might need to upgrade your bottom bracket.From FSA, you can get the Vero Compact crankset, square taper, and will set you back ~$85.Alternatively, you can upgrade to a 10 speed shifter, derailleur, chain, and cassette.Sorry there's no easy answer!

You can get a compact double: http://www.backcountry.com/store/SHI0066/Shimano-America-Ultegra-SL-Crankset-Compact.htmlCurrently it's out of stock from backcountry.com, but your local bike shop should be able to order it in ASAP (just ask for them to place it on their next QBP order, part CR6497 through CR6500).

The largest chainring I can currently find available from Shimano is 55T. However, you can find larger chainrings, i.e. Vuelta sells a 60T in 130mm BCD that would work.