An 8 or 9 speed cassette won't fit a 7 speed freehub but if by a ring you mean there is a spacer behind the 7 speed cassette then you may have an 8/9 speed freehub body. Does the ring look like this?http://www.sjscycles.co.uk/wheels-manuf ... prod18561/
There is a ring like this behind the cassette and also a plastic (big) kindda spacer. So i'm almost positive it's a 8/9 speed hub.
Strange thing is I also have a 8 speed derailleur (Shimano SLX 8 sis) together with an SRam 8 click gripshifter....So now it's even stranger to me it has a 7 speed cassette on it...