some general info.
Since you've decided to "grow" into a HT and use as much of your current system as you can, I'll make the following suggestions.
1) Plan on starting out with a new HT receiver, but playing it in stereo for now. I know, there's no whiz-bang multichannel for now, but there's a method to my madness.
1.5) If you don't have a basic DVD player, get a cheapo one for now. Remember, this is temporary. Blu-Ray is still coming down in price and the longer you wait, the more feature laden and cheaper/more inexpensive they get.
2) Use your two existing speakers for now. Replacing these will be your next step. Yeah, it's still stereo, but you're moving towards a goal.
3) When you decide to go for (at least) your front speakers, look at what you budget for them as covering the two mains and the center speaker. These should match and it's best if they be from the same manufacturer and line. These change fairly quickly and filding a matched center next year may not be so easy. If you don't buy the rear surrounds now, you can move your current speakers to the rear for use as surrounds.
4) Now would be a good time to look into a sub. don't skimp here. This is what delivers 90% of your system's "impact". Oh, these don't have to "match" your other speakers. These are fairly universal and some of the best subs come from people who only (or mostly only) make subs.
5) If you haven't replaced your rears by now, it might be a good time to do so if you feel it's warranted.
5) ...or, if you've made the jump to a HD TV by now, maybe that Blu-Ray player? Without a HDTV, you can't appreciate a Blu-Ray player.