I've got four DIMM slots in my motherboard (MSI MS-7236 Version 2.2, 945P Neo3).
As I understand it the board supports up to 4GB, I've been running it with 2x1GB.
I've tried adding 2x1GB more RAM today.
The old sticks seem to work in either the A or B channel and the new sticks also seem to work in either the A or the B channel, but when I add any more than 2 sticks I get no graphics output at all on booting and I don't get any POST beeps where normally I get a success one.
Any suggestions?! Thanks.
ETA: I bought two more sticks of the same time and it now registers all four (or at least, 3200MB worth of them). Thanks all.