I would like to add that in any kind of cycling your bike will come with a "component group" consisting of things like cranks, brake levers and calipers, fr and rr derailers bottom bracket,etc..
There are two main companies and one new one. Shimano, Campagnolo, and new SRAM.
The component group has various levels, i run shimano, so i will give you that list. It starts at the top of the line called Dura-Ace, then ultegra, then 105, and sora.
So Dura-ace is lighter and more durable than ultegra and in turn lighter and more durable than 105. These kits are not cheap and therefor buy as good of a kit as you can, and sometimes bikes come with a combination of say ultegra cranks but 105 for everything else. Now how much better or more durable the kits are has always been over for debate, but what is not is the weight. The more money you spend on road cycling the lighter your bike tends to be.
Carbon bikes with dura-ace kits tend to be as light as they come, but we are talking that people in cycling literally count the amount of grams in regards to how much lighter something is, when clearly they are still 10-20 pounds overweight. So you will find that people have more money than need in regards to a bike. The way i see a lightweight bike working is that if you have a 15lb bike and the guy next to you has a 16lb bike, then your muscles are doing 1lb less of work moving you down the road. The way the rest of the world sees it, is they all think they are Lance, and want the lightest most expensive bike available.
But I might suggest you start cheap knowing your going to ride for a year or two then upgrade later on, everyone who sticks with it does. Plus its fun as hell to be the new guy on an older or cheaper bike that shows up to group rides kicking hell out of everyone on the $5,000+ bikes!! Because there are alot of 40 somethings like me out there who have really nice bikes who can't use them to full potential until i drop a few pounds.
If you want to have some fun looking at how much everything weighs.
go to
http://weightweenies.starbike.com/