Welcome to the home theater group. Once you get the bug, upgrades are nearly unavoidable. In fact, I don't think you really get a good handle on home theater or audio until you've put together a couple systems. About your questions, the center channel handles about 70% of the dialogue and music in a home theater and if you listen to music in Dolby 3, it's also 70%. This makes the center channel the most important speaker. So, if you are happy with your current speakers, I don't see anything wrong with just getting a new center channel speaker and subwoofer. The problem is that when sound pans across the soundstage, you may notice the mismatch in speakers. This doesn't happen too often, so it shouldn't be a huge problem.
However, many people, myself included, do not use a center but prefer the phantom center setting for DD or Prologic. As long as you normally only have two people in the sweet spot the phantom center can often sound much better than a true center channel. Thus you may want to either keep the speakers you have now or get a new pair for mains and forget the center channel.
As far as subwoofers go, they really do make the home theater experience. If you are planning upgrades fairly soon, it may be best to invest your $800 in a good quality subwoofer that will last you a few years now and get new speakers later. My wife wasn't very interested in my home theater stuff until I got a good subwoofer. Once I did she now loves to watch movies. So if you're married this may be a good way to ensure future upgrades. Check out the TN1225 with the 250W amp from www.hsuresearch.com,
I think it's around $800. I have the ACI Titan sub from www.audioc.com.
It has a much better appearance than the TN1225 and is much more sturdy. It does not generate the high SPLs that the TN1225 does, but even in my rather largish room the Titan is more than loud enough for me. The Titan also is about $800. Both subs allow for a 30 day in-home trial. Get a good demo/setup disc (I use the Delos International disc) and a SPL meter from Radio Shack before you start auditioning subwoofers.