Strange Problem When Trying to Upgrade My MacBook's Memory

I have a 2007 (late?) White Macbook 2GHZ Intel Core 2 Duo 667 Mhz DDR2 SDRAM Serial #W8745CUJZ62 which came originally stocked with 1 GB of memory but states it can support up to 4 GB. I've never upgraded memory before so I looked around online for compatible sticks and reviews and bought 1 PNY 2GB DDR2 PC2-6400 memory stick. I replaced one of the factory 512 MB sticks with this and it worked great. Today, I decided to upgrade the other factory stick so I bought the EXACT SAME PNY 2GB stick. When I placed it in there my Macbook (now there are 2 DNY 2GB memory sticks in) turns on but stays at a black screen. At first I thought the new stick was a defect so I returned it for another one and got the same problem. I did a few mixing and matching paired each DNY stick with a factory one and that combination always worked so I know the new memory isn't the problem. What is the deal? I went to Best Buy and they couldn't figure it out. They even tried pairing one of my 2 GB PC6400 DNY sticks with a DNY PC5300 1 GB stick and that combination gave us the same black screen? Any MacPros out there with advice???

You purchased incorrect memory modules. Here's what Mactracker says:

Maximum Memory 6.0 GB (Actual) 4.0 GB (Apple)

Memory Slots 2 - 200-pin PC2-5300 (667MHz) DDR2 SO-DIMM


Return both of the modules and purchase the correct ones together as a matched pair from either Crucial or Other World Computing.

Thanks for the reply. Memory speak is like foreign language to me...so outside of DDR2 the main thing I am looking for is the "PC5300" part? I am curious as to why when I use one 6400 it works and upgrades the memory to 2.5 GB?

You got lucky. But as you can easily see, as soon as you attempted to put the second module in place, the screen went black. Macs are very sensitive about using the correct memory modules, more so than a PC. Buy the correct modules and it should work OK.