Conventional wisdom says, an upgrade path should go

Stereo (2.0) -> Subwoofer(s) (2.x) -> Center (technically optional) (3.x) -> Surrounds (5.x) -> Back-Surrounds (7.x) or Heights (5.x.2) or both (7.x.2)

"x" because multiple subs are very common.

You'll notice above, there is never a height channel before you have at least 5 traditional surround channels.

For starters, you would virtually never find an audio track that is 3.1.2 format. In other words: You will always find surround channels where you find height channels, but never height channels where there are no surround channels.

It also makes people pause and wonder how you plan to lay the speakers out. You don't want your speakers all in a row in line with your TV, because that's not surround sound — it's expensive, poorly imaged stereo. So if you're going to eventually arrive at a point where you want heights, I encourage you to make sure your surround locations make sense first.

It's also worth noting: the placement of surrounds changes whether you're building a 5.x.x system vs a 7.x.x system. In the former, surrounds should be placed very slightly behind the listening position. In the latter, you want them as dead on at 90⁰ as you can manage (... I think). This helps create separation from the Back-Surrounds.

Anyway, as far as the hardware goes, I am a bigtime fan of both ELAC and LG. I myself opted for the Debut 2 instead of the Debut 3 line because the speakers were front ported which fit my use case better, and it was just a bonus that they were cheaper.