A byte is eight bits. A ninth bit can be appended to each byte for error-checking/correction of the other eight bits. The ninth chip is for this purpose. A 9-chip "16"MB simm is actually an 18MB module (16MB of user bits, +2MB of parity). If you ignore the ninth bit/chip, you have 16MB of user bits, and zero MB of error correcting ("parity") bits. You do not need a module with parity, but having it won't hurt. As equill said, it will be ignored. If you have the option of not paying for it, then save the money. If you don't have the option, then you get what you can get.

Hope that helps.