Careful there, Steve.

It's common for folks to veer off into groupset envy land. Just remember: The only thing you can't easily change on the bike is the frame. And the frame is what gives the bike its unique feel, ride, and handling characteristics. So when bike shopping, stay focused on the frame, not what's hung on it.

That said, Shimano's groupsets rank in the following order: 2300, Sora, Tiagra, 105, Ultegra, and Dura-Ace. There's some variation among the groupsets, but most include drivetrain, brakes, pedals, hubs, and complete wheels. See http://bike.shimano.com/

There's general agreement that weight drops and finish quality improves as you move up the line. Others disagree, but in my experience, there's little difference in shift quality between them. My Sora bike shifts just as smoothly, crisply, cleanly, and quietly as my 105 bike, my Ultegra bike and my Dura-Ace bike. I never feel "less than" when riding a "lower" groupset.

Where I notice the differences are in the brakes, and the levers. The higher-end brakes have a nicer feel and much better modulation. My Dura-Ace levers have completely different--and nicer--feel from the others. It's hard to describe, but they feel clickier.

There are also differences in the thickness of one's wallet as one pursues name cachet.

But when the rubber meets the road, there's no reason to shop by groupset. Shop by frame. You can always play the "upgrade game" later.

EDIT: There is one place where groupset affects shift quality, and that's in the minds of LBS mechanics. Until I learned how to do my own work, I took my bikes to the LBS. I was actually told (by tow shops), "It's Sora. It's supposed to shift like crap." And when presented with the same grievances on another bike, teh same mechanics told me "Yeah, 105's supposed to shift better than that" and they got right on it.

Doing my own work, I've found that similar effort achieves similar results when tuning the shifting of all my bikes. I can do a slapdash job on my Dura-Ace bike and make if shift like crap. Meanwhile, a little extra attention and my Sora bike's shifting is flawless.

And as for durability, I can now officially proclaim that Sora components can outlast the frame they're hung on. My Y2K vintage Trek 1000 has suffered a cracked head tube. I'll be moving its 8-speed Sora components to a new frame.