So here's your situation:
1. You just bought a brand new road bike
2. You already have a case of upgraditis
3. You expect to get into road biking more seriously

Here's my recommendation - take your bike back to the bike store and exchange it for the more expensive model that comes with 105.

I just wrote a whole piece on the costs of upgrading components here -
http://www.bikeforums.net/showthread...=1#post9681168

The short version is that if you do a lot of road biking you will get real tired of the thumb shifters that those Sora shifters have - you can't shift from the drops, and it gets really really annoying if you spend a lot of time road biking. To replace them with Tiagra shifters at full retail price, with the shop replacing them, will likely cost about $300 (I checked recently) - that's starting to approach the cost difference between the 2 bikes and you don't get the other benefits of the more expensive bike like a better derailler. As someone else mentioned, if you're willing to scour ebay and buy older 9 speed 105 stuff AND are willing and able to install it and adjust it yourself it may not be that bad, cost-wise. But most of us neither want to spend the time on it, or have the technical expertise to install and adjust all this stuff ourselves.

It would, in my opinion that's based on little more than looking at the specs, also give you a nicer frame. At the very least a nicer and more comfortable front fork.

On the other hand, if you spend a lot of time road biking you could just stick with what you have, and if you're still seriously road bike jump up to the $2k or $3k Roubaix's which have an even more comfortable and vibration absorbing ride. :-)