I understand your wanting to get speakers that will wow you and stay in the family for a while. You definitely don't want to let your eyes get too big for your wallet... or on the flip side, try to get everything all at once and end up with stuff that isn't much better than what you have now.
If you're going with a projector and screen for a dedicated media room, then you should definitely get a fixed, tensioned screen. You also want light control in the room, and any windows must be light blocked (at least during the movie, or semi-permanently if it will fit a more theater room esthetic)...
Screen size is determined by viewing angle, which is set by seating distance (you can't determine that until you've locked down your room... or "man cave"). Here's a calculator that might come in handy (at least for a 16x9 screen): http://myhometheater.homestead.com/viewingdistancecalculator.html
As I mentioned in a post above, you have to choose if you want a 1.78:1 screen (HDTV ratio, or 16x9) or a 2.35:1 scope screen (constant height), for a more epic look. Check out the 2.35:1 forum for options... and pics. If you will have enough room to put speakers behind your screen, I absolutely recommend going with an acoustically transparent screen material (if you can swing even further, get it 4k resolution ready). Completely worth the added expense and it will lock the audio with the visual information. Seymour is a well regarded screen manufacturer. http://www.seymourav.com/screens.asp
If you do go with an acoustic screen, then you will want three identical vertical speakers (and two more if you want to use DTS Neo:X on your receiver or for the future object-oriented formats that will probably have an option for five front speaker "channels"). So, it behooves you to investigate and get a good theater design plan in mind first before running out and getting the wrong type of speakers.
The Sierra's are quite good for the money and made with excellent music reproduction in mind. But if you start into a more dedicated movie room, you might want to check out Triad, JTR, and Seaton speakers too (just to name three excellent companies) that will reproduce more dynamics (more oomph) and at a greater volume. Granted, if you're not wanting to raise the roof and decide you're more an audiophile at heart, then three Sierra towers for the front and Sierra-1's for the side and rear surrounds would be great too. Then get an HSU or Rythmik or other bang vs. buck sub (or two).
And yes, quality, matching surrounds, now more than ever, are a must. Save up and don't skimp. Speakers are the most important purchase you can make (besides subs and power amps). Most of the budget should go to them.