Couple of things since I have the same sort of setup as you.
(1) M2N-SLI Deluxe is good for OC on paper, but in reality it is not as it's voltage has been messed up since the first revision of the board and the very first BIOS. This was never fixed. You can volt-mod your 5000BE if you want to run at 1.55, but I have a feeling that it would have problems on this board. Otherwise, pretty much max voltage you are going to get will be around 1.44V. Basically, even setting the voltage in bios to 1.5675V, will only give you about 1.44-1.45V actual. This is a known problem on this board, at least to everyone who tries to OC.
(2) I am currently running my 5000BE stable at 3.27GHz (218x15) with HT multiplier at 5x and memory set to DDR2-800 4-4-4-12-2T (see rig below). I had it stable at 3.3GHz but I backed it off a bit, just in case. To get this OC however, I had to remove the IHS from the CPU. Before I did that, highest stable I could get was 3.15GHz.
By the way, I also have the Brisbane core, and 4 DIMMs at 1GB each
Oh and see the attached screen for voltage details (temps are not correct, but I run at 31-35 idle, and 45-55 load).