As Tony said, one of the biggest determining factors will be the number of records that you have. In addition will be the type of calculations you want to perform on your data.
Access is hugely powerful and works great in conjunction with Excel. I.E. I work with up to 1 million records in several Access databases containing weekly sales information from our AS400 orders database. Access allows me to take that base level sales information and compile it up to Sales Rep, Sales Manager, Region, Market levels, etc. From there I can work with it in Excel and perform complex calculations.
You can perform complex calculations in Access, but it is often nowhere near as easy as in Excel. Excel is a much more robust analysis tool, while Access is a great relational database.
To pull Access information into Excel goto Data-->Get External Data-->New Database Query. Select Access from the list and follow the wizard.
To get Excel information into Access, goto File-->Get External Data and follow the wizard. Note that Access can be very picky with field types (Number, Text, etc.), so you have to be careful with whether you choose to Link Tables or Import them. A linked table in Access will reflect changes made in Excel, while an Imported Table is static and has to be re-imported if changes are made to the original Excel table.
Hope that helps, but post back with more specifics.
Smitty
