I have the same problem.
I bought 4*512mb single sided memory, each has 8 IC on it.
So.
With 2 memory, in 1-3 DIMM slot, easily boot up.
With 2 memory, in 2-4 DIMM slot, easily boot up again.
With 3 memory, in 1-2-3 DIMM, boots again.
I make a test, with memtest 3.3, when 4 memory was installed...