The faq 'small change' links to is a great place to start and I'd like to second the comment about buying second hand or last year's stock. At this time of year all the 2006 bikes are out, so it's a good time to pick up a 2005 bike at a knock down price.
If you have somewhere local to you that rents out bikes by the hour and they have reasonable bikes to rent, try one and see how it feels.
If you're after something light are you thinking of racing someday?
