Oof. I have been having some horrific issues with kernel panics on my early 2008 MacBook. I ordered 4GB of RAM from MemoryX a few weeks ago. I installed it and ever since then I've been getting increasingly common kernel panics. The first happened when I was running Minecraft, which is a buggy piece of shit still in beta. I've had several more while watching YouTube videos.
I downloaded memtest and booted into single user mode. I tested it first with one stick removed, then the other, using "memtest all 2" as prescribed. Both sticks passed fine. Last night I did it again with both sticks installed, and again the test passed completely. A friend of mine suggested it might be an overheating issue. The Macintop's fan runs at full speed when I play back FLV or run Minecraft, but that is to be expected. I had the machine cleaned out a year ago when I took it in for AppleCare servicing. There's some possibility the air path is blocked, but the laptop never feels excessively hot to the touch (just the same temperature it always gets when running processor-intensive apps.)
I had another kernel panic today while nothing but FFX, mail, Adium and text edit were open. That's not a huge stress on the system. I'm kind of lost as what to do at this point. My model MacBook was designed to work with 4GB of RAM. Tonight I'm going to try an extended run with memtest on both sticks. Is there anything else I should be trying?