Sum if?

Hi, I'm trying to have excel 2007 sum a value in one column if another column matches the value.

Here's something similar to what I have:

A | B | C
Item | Order # | Cost
Widget | 046 | $50.00 |
Thingamagig | 047 | $100.00
Dohickey | 046 | $70.00

I'll have multiple items with the same order number, and I want excel to give me a total of the cost column for all items that are from a certain order i.e.

Order 046 Total: $120.00