Sum multiple results from a vlookup based on dropdown value

Hello,

I am wondering if the following is possible:

In a workbook, I have a cell called Result which sums a few numbers (amounts in $ in this case). Above it is a dropdown containing percentages (25%, 50%, 75%, 100%) which should influence the sum of the Result cell.

Is it possible, for example, to set the dropdown value to 25% and automatically perform a lookup for all rows containing that 25% value, fetch the values contained in the Amount cells associated to those rows and add them to the original amount displayed in the Result cell?

Since I am having a hard time putting my question into words, I have attached a sample workbook explaining what I am trying to do. The cells of interest are colored in orange/yellow. At the bottom of the "budget 2009-2010" worksheet is a cell called Result with a dropdown right above it. When changing the dropdown value, I would like to get all the values in the Amount column of the Opportunities worksheet where the Probability (%) value is the same as that of the dropdown in the "budget 2009-2010" worksheet and add them to the Result cell of the "budget 2009-2010" worksheet.

Any help will be appreciated!