Dimension E521 with 2 HDDs - How to reinstall Vista so that both HDDs are used?

Hi,

I have a Dimension E521 that came with 2 x 160GB HDDs. One had broken completely and the other was on the way out.

So, I decided to replace one or both HDDs. As I had been running happily on one HDD for some time I thought it may be possible to simply buy and install one HDD and reinstall my Vista.

Firstly however I had a chat online with a Dell representative who told me, having given her my service tag, that I must replace both HDDs.

So I bought the 2 replacements and installed them and installed the Vista os. To my surprise the 2nd HDD has not been involved at all.

My questions are:

1. Was the advice wrong - ie. can I simply use one disk?

2. If I do need both disks, is there something more I need to do to get the system to involve the other disc.

3. What is the other disc for anyway?

Thanks

Mike