In reply to Slugain Howff:
The Trek 4900 was a pretty decent MTB in it's day from memory and I suspect the kit on the Carrera is not much different from what you've got on the Trek!
Best bet is to get yourself to a few bike shops and talk about riding position etc. Buying from a bike shop is expensive as they tend to charge full whack for the bike compared to online, but it can save time and effort in the long run getting a good fit.
If you do buy online, bikes labelled as 'Sportive', 'Relaxed Geometry', 'All day comfort' etc tend to have slightly more upright positions which work better for commuting comfort.
There are three main drivetrain manufacturers in the world of road biking: Shimano, SRAM, and Campagnolo. People will argue which is best, but they are all good. They all produce a hierachy of components right through from basic to professional quality. Looking at what drivechain components a bike uses is often a good indicator of it's general quality. For example, shimano produce 5 ranges.
Sora - entry level, does the job, still way better then cheapo but a bit clunky
Tiagra - budget but still not half bad
105 - probably the best bang for the buck
Ultegra - most popular with amateur racers and enthusiasts
Dura-ace - used by the pros and people with a lot of money
They also produce stuff such as Tourney solely for the Halfords market - best avoided.