OP...how tall are you? Do you have long arms and legs?...or shortish?
Also, if you get a chance, test the Cervelo S3 if you like to ride fast. Many prefer the S3 over the Cervelo R series bikes because there is little detriment in ride quality relative to the aero advantage of the S...many times not the case with aero bikes. Cervelo did a very good job on the S series.
The Tarmac is a fantastic bike and many consider it the gold standard among race bikes.