All memory is not the same.
I cannot just go out and get a module of either low-density or high-density PC3200 for my system and expect it to work. It may, it may not.
Too many people don't realize that there are differences above and beyond just the type.
FWIW: I experienced this very same thing yesterday, with a new motherboard/CPU that I had sitting around for about 8 months (I'm seldom in a hurry). All fans, drives, everything works...but the monitor says "video cable unplugged?"
After trying a couple of other vid cards, I reasoned that it's the memory that I was trying to use from another system. I'll buy some this weekend that ought to do the job. If not, I'll have spares
PS: The RAM I tried is now back in the system I am using.