Early 2008 Macbook (White) Memory Upgrade Problem

My early 2008 Macbook (white) originally came with 2x1GB of Samsung memory. I am trying to replace these with 2x2GB, which is the maximum amount the manual says it can handle. I purchased a set of PNY 2x2GB DDR2 PC2-5300 667MHz cards. PNY's product number is: MN4096KD2-667. When I install both new 2GB cards, my computer does not boot. The power comes on, and the HD revs up, but the screen stays black. If I have one new 2GB card installed with one of the old 1GB cards, the computer boots without an issue.

Here's the overview of my computer. As you can see, it recognizes 3GB of total memory (one old 1GB and one new 2GB):

As far as I can tell, the specs for the new memory are consistant with the requirements provided in the manual from Apple. (DDR2 PC2-5300 667MHz, up to 4GB total) Is there a reason why my computer can use 3GB of total RAM but not 4GB? Since my computer can recognize the one of the new cards when paired with an old card, it would seem the memory is compatible with the computer. I must be missing something.

Has anyone else had this problem? If so, were you able to resolve it? How did you get it to work?

Thanks in advance!