I also use rear surrounds and use PLIIx on content that isn't 7.1 and it works pretty good. The effects aren't always obvious but I think they're worth it. A lot of the tv shows on my Netflix (first season of Stargate SG1, Firefly, etc...) are in stereo and I can't believe how well PLIIx works. On top of that I'm using old speakers from my HTIB for the back until I'm allowed by my "boss" to buy some extra QS8s. Nonetheless it still sound decent.

On the other hand if cost is an issue then starting with a 5.1 is more than adequate, after all there's not a lot of content with true 7.1 yet.