The only time you're likely to ever really notice a difference is if you have a game with a discrete 7.1 audio track. I would imagine right now probably 90%+ of the time you're using upmixed simulated surround, so I highly doubt you'd ever be able to tell the difference unless you're a serious audiophile, and how likely is it that you'd even ask if that were the case?
The 7.1 basically adds two more speakers to the back, instead of between back and side, so it really doesn't make much difference since on a 5.1 the two back/side ones will balance out to make it sound like its behind you.
Sure it's worth it if you have the money and space for seven speakers. Do you notice a difference when you go to the theatre and watch a movie that's been recorded in THX vs. DD5.1? I do. danawesome89's advice is good as well. Invest in good speakers, no matter how many you end up using.
I can tell the difference between stereo and 5.1 with my headphones when gaming.
Yeah, I said that there is hardly a difference, not that there isn't a difference. In my opinion 5.1 headphones are not really worth it, and it is much better to just get really high quality stereo headphones.
With actual speakers, it is a whole different story. Then there is a huge difference between 5.1 and stereo (and a much smaller difference between 5.1 and 7.1).
So basically my point was that interpreting surround sound quality for speakers based on headphones is not a particularly effective comparison.