Ok to get it clear for everyone
7 and 8 speed systems are totally compatible, except you will
miss one gear (when 7sp shifter with 8sp cassette) or you have to
adjust the derailleur range (when 8sp shifter with 7sp cassette)
9 is different, there are nine cogs on the same width as the 8sp, so the
cogs and the space between the cogs are smaller. explaining the
ridiculous rate of wear of 9sp systems. It also explains way 8 and 9 sp
shifters are not interexchangable; the 9sp cable releas is smaller.
But 8 and 9 are compatible in another way: the cassettebody is the same,
you can interexchange the crankset, the front and rear derailleur (alltough
9sps' tend to be a bit narrower).
10 speed cassette is again based on the same width, but they don't always
fit onto the 8/9sp bodies! just to make it more annoying I guess. But I was
already resent the 9sp so don't even bother to go near the 10.
But if you go for an indexles thumb shifter you can put in 7,8,9 even 10
speed wheels in your bike without changing your shifters