+1 on giving everyone more time to respond. The other thing that I would mention is that if you don't get answers as quickly as you'd like, perhaps you should check with your nearest bike shop.

Another thing that I would mention here is that unless your bike or current chain/cassette is fairly new, you may have to change your chain as well. Your chain and cassette mate together, and replacing one without other may cause serious problems.

I agree with others, that perhaps a bit more information about you, your riding habits, terrain would be helpful. Hopefully all of the responses have guided you in the right direction.