I had an industry colleague today try to say that they could not expand their linux machines (indivdual or clustered) beyond 2 GB of RAM on a 32-bit kernel. I was almost positive that was not true, but I did not have any numbers to quote him. I'm looking for the maximum RAM that certain kernels can be configured for. For example, what is the maximum configurable memory for a ...
2.4 (32-bit) kernel,
2.6 (32-bit) kernel,
2.4 (64-bit) kernel, and
2.6 (64-bit) kernel?

I suspect that my friend is using an out-of-box 2.4 kernel from an older version of Redhat, which may be limited to 2 GB.

I think 32 bit kernels can have 4 GB if the right options are compiled in. I think 64-bit kernels can go up to 1 TB... Again, you probably have to have the HIGHMEM option compiled in...