In reply to Phil79:
Ive got 10 speed veloce on my good bike and 9 speed veloce on my winter hack.
I've replaced cassettes on both bikes and as far as I can see the main difference is the width of the plastic spacers between sprockets.
Never heard of a sprocket snapping - though back in the 60's I have seen some old 6 speed freewheels with very worn teeth! Depending on the make of freewheel, you could buy individual sprockets if you found one or two were getting particularly worn. It was a lot cheaper than having to replace the whole freewheel - particularly if you were relying on a paper round to keep your bike going.
Nowadays you cant generally buy individual sprockets - even if you want to make a 'custom' cassette when the manufacturers standard set does not suit you.