Speakers are probably the biggest single factor - they're the devices that actually reproduce the sound. I have seen some suggested divisions of budget (don't recall the specifics at the moment, I'm afraid) which put a goood emphasis on speakers. One important step is to demo some different speakers and let your ears be the judge, as speaker buying depends quite a bit on personal taste. A good power amp can also be a long-term investment, even if the processor gets changed out to support emerging technology (as sluggo points out).

If you are staging, I'd definitely lean toward getting your from soundstage now and adding surrounds over time. If you want to get into surround quickly, you could use some existing speakers or pick up something really inexpensive for temporary side surrounds. If your mains are floorstanders, you could even postpone the sub - I didn't add a sub until almost a year and a half after I bought my speakers, and if your mains can get down to 40Hz or so you won't lose much by delaying the sub purchase.