take that shit back and exchange it if possible or try what Neshani was suggesting
I had a problem like this recently. I got 2 1gb sticks to replace the 2 256mb sticks that were in my old ass laptop. I swapped out a 256 for a 1g but only did the external laptop slot on the bottom. The computer would boot and freeze. Once I took the laptop apart and swapped out the 256 stick under the keyboard for the 1g stick that matched my other 1g stick it booted and worked fine. It was a memory mismatch problem that had my computer clucking out even though both sticks were rated the same. It took a few hours to figure that shit out and I was to the point of sending the memory back claiming it was fucked up before I decided to try using both new sticks which fixed my problem.