6'6" beginner rider wonders: 62cm or 64cm Trek frame?

I'm a tall beginner rider preparing to buy my 1st road bike. I've
begun training for a century ride on my Trek 5400 MTB while I shop for
something faster.

I'm considering a Trek 1-series and for 2010 they offer a 64cm frame
in the Trek 1.5 bike. I was wondering if anybody could let me know if
the 64cm would make a noticable difference over the 62cm frame. I may
not be able to justify the increased cost and may end up just getting
a Trek 1.1, but I'm concerned about comfort and getting sore on long
rides. If starting with a larger frame would make a big difference,
I'll try justifying the additional expense.

I'm also wondering how much better the Tigara shifters are over Sora
and 2300's. I know veteran riders can tell the difference, but my
last road bike was a Schwinn in 1989... would I be able to tell?