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Preface: My background is mostly windows environments, so I may be missing something obvious.

On my OS X 10.6 machine when I have 64-bit Kernel and Extensions turned off (which I gather is the default), my computer currently reports 5 GB of RAM in the following configuration: 

DIMM 1: 1 GB
DIMM 2: 2 GB
DIMM 3: 1 GB
DIMM 4: 1 GB

When I enable the 64-bit Kernel and Extensions by rebooting and holding down the '6' and '4' keys, it report 3 GB of RAM in the following configuration:

DIMM 1: 1 GB
DIMM 2: 2 GB
DIMM 3: 0 GB
DIMM 4: 0 GB

Does anyone have any insight into what's going on? How can I get my machine to recognize all the RAM? Is this a hardware problem? It seems unlikely.

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Could you please swap the RAM around (like the 2Gib into slot 4), so that we can exclude problems with the mainboard (driver)? Under the condition that opening the case will not void any warranties or so... – Bobby Mar 30 '11 at 15:56

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It seems that DIMM 3 was bad or was going bad or went bad. I swapped 2 and 3 and rebooted as per Bobby's suggestion. DIMM 2 then reported itself as empty in 32-bit mode. Put the 2 GB back in DIMM 2 and moved the 1 GB to DIM 4. Now both 32-bit and 64-bit report a 1-2-1-0 configuration.

Odd that the 1GB DIMM was still viable in 32-bit but not 64-bit consistently, but now it does not work in either and somehow that prevented the 4th DIMM from being detected.

If anyone has insight into "Why?", I'd be interested.

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