In general frames and wheels dominate the weight of a bike and certain components matter more than others.
Check out:
http://weightweenies.starbike.com/ if you want to compare components or frame weights or see how far some folks have pushed the light weight thing.
or play with the calculators on sites like this one:
http://pedalforce.com/online/your_computer.php?cPath=21_99999_100852_101073
which allow you to do some "what if" comparisons of built up weights based on different components for the same frame.
Going from classic steel to one of the modern thin wall steel, aluminum or titanium frames can save you a couple of pounds. Go to a light carbon fiber frame and you can save another pound or so. A light set of wheels can save you some more with tubulars still running a bit lighter than clinchers. Then you can save more by selecting a high end gruppo or custom picking ultralight parts. Some parts like carbon bars vs. aluminum bars tend to be the same weight so you can't just assume that fancy materials are lighter.
Anyway even if you're very lean and fit you still have to consider the bike weight as a percentage of the total package weight if you want to estimate the performance advantage. When you climb you lift both your weight and the weight of the bike uphill. Same during acelerations, you have to acelerate both your weight and the weight of the frame. Play with the calculators here:
http://www.analyticcycling.com/ForcesSpeed_Page.html
if you want to see what a kilo of bike weight difference makes given your body weight. There's definitely a performance advantage to be gained with a lighter bike, but probably not as big as most folks think.
FWIW I raced all last season on a vintage 21 pound steel road frame I've had since the mid '80s. I had a number of top ten and podium finishes and don't feel the extra couple of pounds made any difference. At least I was able to make the key splits on hills or out of fast crit corners with the extra weight. I upgraded to a lighter frame just as the season ended. I really like the new frame and it's fun to ride but I'm not breaking any records or tearing up the local hill climbs with a few less pounds of bike weight. I sure like the bike but don't have any illusions that I'll suddenly ride off the front on big climbs. Anyway, light bikes are great, but it's still the engine that matters.
-Dave