I'm with all of the above but would personally follow David's path. The two main fronts will create the most impact and give you the biggest bang for your buck.
1. Spend the most money you can on them and build the rest as you go.
2. If you entertain others a lot or sit off center (outside of the sweet spot,) then get a center channel next. If it's just you, then I'd add a sub first instead of a center.
3. Add whichever you didn't get in step two.
4. Surrounds. It may surprise you that we all leave these until the relative end but the majority of movies are just starting to get produced with a lot of decent surround effects. Even with a big budget action movie where they get used a lot, I'd rather have great sound and clear dialog up front.
Another method to consider might be to still get a pair of really good mains but leave just a couple of hundred for a cheap HTIB speaker system to tide you over for the rest of the components until you replace each one over time as you have the money to upgrade them.
The nice mains will give you your bulk of quality sound and part of the fun as you progress will be hearing what a huge difference it makes as you swap out the cheap little satellite speakers with better ones.
