The only way to get 7.1 is to use one of the matrix decoders that can create 7.1 from 2 channel, 5.1, or 6.1 sources - like ProLogic IIx.
If your room is large enough and you can put the rear surround speakers well behind you then 7.1 can help to create a more enveloping sound. Some others here who do utilize 7.1 say it does make a difference in some cases; I can't say for sure because I don't use it. IMO, for most rooms, 5.1 is fine.
The other question not directly asked but related is whether or not one should buy a 7.1 receiver even if they only plan on running 5.1. I think the answer to that question is Yes for two reasons:
1). Many 7.1 receivers have a zone 2 capability whereby you can use 5.1 in room and stereo in another at the same time (playing different sources). That is useful if say you want speakers on your patio.
2). You have the ability to 'upgrade' to 7.1 if you so choose. Regardless of how many speakers you are using at any time, you can tell the receiver which speakers are connected so the number of channels in the receiver doesn't have to match the number of speakers.
