Data table will let you specify several different choices for a value and allow you to compare the results. Scenario Manager allows you to do pretty much the same thing but captures each version separately. Goal Seek will automatically find an optimal value based on some outcome that you specify, and Solver does something similar but allows more complexity in the specification of your problem.

Lots of information about each of these tools is available in Excel Help and on the web. If you give more information about what you are trying to do, and better yet attach a sample file, we can give more specific advice about what will best fit your situation.