Last week I dusted off my old Trek SU100 hybrid/commuter and did a 20 mile ride around san francisco... and caught the cycling bug bad. Since then I've been pouring over reviews/bikes/options for a pure road bike, but the question that is the hardest to determine is which components I can skimp on, and which I should splurge on. I realize everything important, but with a limited budget, I'd like to know I'm focusing on the right things. To use an analogy from a different discipline: When I buy a computer I know I can skimp on the graphics card (I don't game), but need to get the best/fastest HD (lots of I/O).

A little about me to help:
5'8", 180lbs.
Primary exercise is spinning (3x week usually)
Plan to use the bike as a weekend warrior/fitness & group rides.
Only weekends, and only long distance (min 20miles) little to no commuting.

So for example, I've gathered that Shimano 105 shifters are much better than Shimano 2300's or Microshifts - but in terms of priority, is that like the second thing I should be concerned about or the 10th?