So it's been a while since I invested in some gaming speakers, and while I WANT new speakers, I don't need them. That said, the last set I bought were the venerable Z680s. To this day, I am blown away by how much bass comes out of this subwoofer, the clarity that I hear from the mids and highs, and the flexibility the speakers have in their dynamic range. What are the high end speakers? Last I checked, it was the 5500s or the Gigaworks.
Z550s. But, I doubt you will need them, they won't be a huge amount better than the Z680s you have now.
I don't think either the Z5500s are the S750s would be worth it over the Z680s. The Klipsch 5.1 set it meant to be good tool, but again I'm not sure if you're going to see vast improvements over Z680s. You could always go for AV stuff though. If you're willing to buy second hand you could put a full 5.1 setup together for not much more than the Z5500s.
gigaworks are amazing, but the amp on them dies every year... or at least mine do.. been through 4 of them!
I don't think there will be a huge difference between the Z-680s and the Z-5500s, but if you're set on buying new ones, I really do enjoy my Z-5500s for 5.1 gaming.
I have to echo _tad - you'll get a better-sounding seperates system for the same money. It might not be as loud, or have the bling factor, but it's definately worth doing.
Listening to a friends set of speakers and selling them in the shop all the time they are the best gaming speakers I have come accross in a long while for a cheap price ~Kraeg
Aego M http://www.acoustic-energy.co.uk/Product_range/Aego_series/Aego_M.asp I have these and I love them. Deep crisp base and the clearest sound I have heard from a PC. If you get a chance try them and I am sure you will like them as much as I do. They also do some surround sound speakers, as you can see on the web site.
have you ever thought forget 2.1 and go with a real set of speakers like these http://www.htfr.com/more-info/MR243596
If you are going down the active monitor route then something like the Tannoy Reveals are supposed to be rather good. That said, I think the OP wanted a surround set?