Automatic date calculation

I need to be able to setup a spreadsheet that allows me to enter a date then in the next cell enter a priority ( preferably from a drop down box ) and in conjunction with that priority insert another date in the next cell,


Eg.

2//7/2010 - Priority 2 - 6/7/2010


Priority 2 representing 4 days.


So the user can obviously select the entry date and the priority of the task and a date will appear in the next cell or column with the completion date.


Priority List is to be;

1 = 2 days
2 = 4 days
3 = 6 days
4 = 14 days
5 = 28 days


Thankyou very much to anyone who finds time to help me as out of my office no one can seem to figure it out!

Cheers.