Most triathletes in my area have doubles, and I thought I wanted one too. (But I'm not interested in a tri-specific bike.)
But a Giant salesman told me that the gearing on the TCR Composite bikes could be changed so that there would be virtually no difference between their double and the a triple. As a novice, I don't know how I would figure out if that's true or not. Are there some web pages out there that explain gearing? Not to mention how to translate the specs on the manufacturer's web page. Thanks for your help!

Sounds like the salesman is trying to pull the wool over your eyes - maybe he doesn't have a triple to sell you. Yes, a double and triple can be set up so that there is virtually no difference in gearing. But, the problem with this is it would not be set up to take advantage of the lowest possible gearing combinations available with a triple. If you REALLY need those lowest 2-3 gears the only way is to get a triple. There is NO way a typical double (53/39) can be geared as low as a typical triple (52/42/30) - it is numerically impossible. An FSA Compact crankset (50/34) can get even closer to those lower gears, but obviously the "30" ring is still smaller than the "34". The question though is, do you really need those extra couple of gears? Usually the answer to that lies somewhere in the terrain/altitude you ride and your general physical conditioning.

There have been many discussions on the general advantages/disadvantages of double vs. triple and you can search this site to find those. Here's a link that will calculate gear ratios for you so you can see the numerical differences. Sound like you should ride a triple set up with the lowest possible gear combinations so you can see how it feels and if you think it would be necessary. I'd like to add more info but I'm out of time. Maybe I can come back later if no one else has expanded on this.....hope it helps a little.

http://www.panix.com/~jbarrm/

Thanks very much, your info was very helpful.