5.1 is only worth it if you actually set it up as 5.1 (meaning 2 front, 2 rear, center (5) and subwoofer (.1) = 5.1 lol. The 5.1 won't make a difference to music really, as it's usually in stereo. Movies benefit from 5.1 and games can too. Personally I don't have space to set up my rear speakers. I find 2.1 to be very convenient, sounds great and I'd rather have 2 great speakers and a subwoofer than 5 okay/possibly cheap speakers and a subwoofer.
It's not hard to set up 5.1, it's tricky to fine tune your surround sound though. Need's a good sound card.
Don't go for a 5.1 and put all speakers on the desk, completely pointless. Unless you run the satellite speakers in double stereo, 2 of them won't be doing anything, and if they are you will be hearing things behind you in front of you. The speakers are 2 for front and 2 for rear. Just foolish when people do this. If you will have speakers on desk PLEASE for the love of god get a 2.1 stereo system. Also you will have loads of unnecessary cables.
You wouldn't buy a pair of headphones with all the speakers in 1 ear. So makes no sense to have rear speakers in front of you.