Ubuntu :: Karmic 64bit Boot Hangs With 4GB In Machine?

I have Karmic 64 bit installed, originally with 2GB of memory. The motherboard will take up to 16GB, and I decided to double to 4GB. When I put in the 4 memory sticks (1GB each) and reboot, it gets past grub2, to the ubuntu "circle" symbol, the screen blacks out and that is it. No desktop, no further action that I can tell from the box, just a black screen. The video feed to the monitor, from what I get with the power light is gone and no activity on the drive.

If I take out one of the memory sticks, it boots happily up with 3GB and runs fine. Put the 4th back in, same behavior. I tried swapping out the old memory, putting in the 2 new sticks, everything works fine. Add in the other two (the old ones), the problem shows again. I thought I might be the last memory slot, so I removed the one from the 3rd slot, works fine. All in all, it works fine with 3GB, but hangs on boot with the 4GB.