You'll definitely need a new front derailleur when switching from a double to a triple. A double front derailleur doesn't have the right range of motion to get from the inner ring to the outer ring on a triple. To tell the truth, the only changes you need on your bike to switch are to replace the left side shifter, a front derailleur and possibly a rear derailleur if your current one doesn't have enough capacity.
I'll probably finish the upgrade by the end of next week. Just need to get a 10 speed chain and then I'll put the pieces on. I figure around 10 bucks for the derailleur or just buy me a beer at the next chainlink bar night or after a CM
i will be at critical mass this friday (at least i'm planning on it) but i probably wont have my geared bike with me. what bike (make/model) do you have? i can start some research on my end. i'm not a professional mechanic but i put together both of my current bikes myself for the most part, so i'll see what i can figure out. feel free to email me back at firstname.lastname@example.org
exchange for triple: i think mine is an 04 (when it still used a 9 speed cassette), and my front derailleur is tiagra, but the shifter and crank is 105 and in black. let's figure out if what you have and what i have are compatible (without too much additional cost). -willy