Logitech's high end surround set went
z-560 (4.1) --> z-680 (5.1) --> z-5500 (5.1)
their midrange went
z-5300 (5.1) --> z-5450 (5.1) (yes, I'm putting the z-5450 in the midrange section despite having some "features") --> g-51
their low end went
z-540 (4.1) --> z-640 (5.1) --> x-530 (5.1) --> x-540 (5.1)
Especially in the midrange tier, the models changed significantly, but there's been something between the low end and flagship sets since the z-5300.
The quality of the components in the z-560 to z-680 to z-5500 sets have been fairly consistent with some improvements in areas and some weaknesses as things have changed (in the thread I linked to, there is some more detail on this).
The z-640s were always low end. Going from Logitech's lowest 5.1 set to their newest 5.1 set is not going to be an indication in the difference you'd get going from their old flagship model (z-560) to their new flagship model (z-5500).
The z-680 was essentially just a 5.1 version of the z-560 components along with digital inputs. The z-5500 was a redesign but apparently the satellites are not as good but the subwoofer may or may not have been an improvement depending on who you ask.
Going from z-640 to any of the flagship models would of course be a night and day difference.
The z-560 to z-5500 move would make sense if
1. There's a definite need for digital inputs
2. There's a need for a center channel rather than placing the 4.1 set up well and getting a "phantom center" effect from the 4.1 set
If the reason is sound quality, then z-560 to z-5500 would probably be a lateral move rather than the "big difference" going from the low end z-640 to z-5500 experience described above.
To make it worth the effort to upgrade from the z-560s sound quality, I would recommend looking beyond computer speakers to get away from some of the problems common to all computer satellite speaker systems.
UPDATE:
So SniperDaws, your response after reading my comments is to change 580 to 680? That's it?