negative1 said:

is there a noticeable difference

between 5.1 and 7.1 sound?

If I may answer this, I'd say that it comes down to your viewing environment.

If you have a wide configuration, the additional 2 rear speakers (originally 1, as in 6.1, until they realized that having a single speaker directly behind you could often give the false impression that the sound was actually coming from infront of you) can fill out the rear stage somewhat.

If your room is narrow, then I don't think the extra surrounds are necessary. 2 speakers, evenly placed and calibrated will fill that space just fine.

I'm not a big fan of having the primary rears at the side of the layout, roughly in line with the seating positions, as is often suggested. This isn't compatible with 5.1 mixes and is less enveloping. I tried out this configuration for a few months at my old place before taking down the side speakers and going back to 5.1, which was much more immersive. At my current residence, where I've once again gone 7.1, I have all four rears behind the seating position (the 'sides' only brought forward slightly) and that works well.

I guess its similar to when people are forced to put their couch up against a wall and must have their rears on either side - its still enveloping but you never quite get the sense of the audio originating from behind you.

Anyway, that's my opinion. Others' may vary.