If that's the case then the Megaworks would be good too if they can be found at all. I find the Megaworks more versatile than the Z-680 (the later being better for movies/games than for music).
On a side note, the Gigaworks S700 is identical to the S750 minus two speakers (which you may optionally acquire later). If I am not mistaken, the G500 lack the tweeters, one of the thing that made the S series great (at least compared to other surround MM speakers). Still, if the G500 is anything like the Megaworks, then they should be very fine too.