I'm with ovoleg. If you want speed, work on your fitness and set up your bike very carefully. An amazing percentage of people with great bikes don't have them adjusted properly.

The actual speed difference between a $1500 and $3000 bike is small enough that you won't notice unless you race and are just barely failing to achieve the results you want. Having said that, if you climb a lot, saving 2-3 lbs can be nice. I have also found that I like the handling on the spendy bikes more.

One thing to keep in mind is that a $3K bike costs more to maintain/fix. Also, crashing becomes a very expensive proposition. My wife once saw me wipe out on my racing bike and remarked that it looked like I was trying to protect the bike as I was going down. That's exactly what I was doing -- I'd rather be bruised and scraped more than ruin an expensive frame or components.