The 600X should be able to accept low-density 256MB modules in both sockets. Maximum system memory is 576MB (2 x 256MB + 64MB). As you seem to indicate, 256MB modules which have 16 components (8 per side) are the ones which the 600X requires. They must also be at least PC100 or PC133 speed rated. The memory modules can only be installed one way; there's a notch along the edge of the module which needs to match the one on the memory socket. If for some reason you are able to install the module in both directions, then that means the socket inside the laptop is broken and I would not use it until you can determine the proper orientation of the module. It's very rare, but it's possible that the memory socket can be damaged in such a way that would allow it to ignore the position of the notch.
It's possible that one or your 256MB modules is defective. Download a copy of either the Microsoft Memory Diagnostic, which I believe you have done in the past, or you can also use memtest86+ (a similar memory verification utility). I would proceed as follows:
- Remove both memory modules and run the memory diagnostic on the built-in memory (64M). I would suggest letting the program finish at least 3 complete passes using all of the standard tests. For 64MB in your 500Mhz 600X, my guess is that will take about 10-15 minutes. 256MB will take 4X longer to complete.
- Assuming that passes, install ONE of the 256MB modules and repeat the testing.
- Again, if that one 256MB modules passes, then move it to the other memory socket and repeat testing. What you are trying to do is verify that that particular module works and that both memory sockets are operational.
- If that module checks out fine, then replace it with the other 256MB module and repeat testing, again installing it in both sockets.
Also, for completeness, can you provide the TYPE number of your 600X (e.g., 2645-XXX) and the part numbers of the two 256MB modules. It sounds like you have the right combination of hardware but there may be a problem with one or the other.
(Microsoft memory diagnostic)
edit: Completed missing sentence in last paragraph (right combination of... was all that I seemed to have posted earlier).