As a newbie- a new bike won't help you nearly as much as getting your efficiency dialed in. My advice would be to run down this checklist (and everyone else's advice), commute on your current bike for a year or more, at least, then be in a good position to decide if a new bike is for you. (FWIW- Bicycling Magazine praised the hell out of a Schwinn Fastback in their annual bike review issue.)

- Make sure you're fit properly to the bike- if your seat is too high or low, you'll be wasting tons of efficiency. Get that saddle height dialed in, and think about the stem length as well.
- Have you tried clipless pedals? If not, then think about it.
- If you're comfortable, go one level skinnier on your tires- 28 seems to be a sweet spot for many commuters. Keep them aired up at all times.
- Keep the drivetrain clean, brakes firm, etc.

If you're mainly riding in the 42T ring, then you still have a whole 'nother chainring to work up to- that'd be the same regardless of what bike you have. Dropping 5lb off a bike isn't going to get you pushing 52x19 much sooner.

Work on the engine- if you upgrade too soon, you'll kick yourself in a year, when you have a better idea of what's important to you. (ie- maybe your best bet long term is a really good cyclocross or decent touring bike, possibly not from one of the major bike mfr's, but at this stage in the game you might be easily talked into a Madone/Tarmac/etc simply because it's on sale and that's what an LBS is pushing at the moment.)