Before you go buying anything, you need to figure out how many sticks are bad.
You can get either memtest86+:
http://www.memtest.org/or the Windows Memory Diagnostic:
http://oca.microsoft.com/en/windiag.aspTo figure out which stick is bad might take you a little while. The general way I approach it is to take one of the sticks out so you can test each stick separately. Basically, test one stick in both of you memory slots, and then test the other stick in both slots. That way you can figure out if it's your memory that's bad or if something is going screwy on your mainboard.
If that doesn't make sense, this should:
You have two sticks (1 and 2)
and 2 slots (A and B)
Take out stick 2, and test stck 1 in A and then B and see if it fails in either slot.
Then take out stick 1 and put in stick 2. Test that in A and then B and see if if fails in either slot.
That'll let you know which stick is bad (if a stick fails in both slots), or if it's a slot that's bad (both sticks failing in one slot).
