If I've understood correctly, the bank has RH Linux (Not RHEL!) as a server, and this is some way serves (Samba? Nfs?) to some clients which probably aren't Linux boxes (Windows?).
RH 4 must be from some time in the late 1990s (this is where it is important whether it is RH or RHEL...RHEL 4 would be only a couple of years old and would probably still be supported).
I'm assuming, though you don't say this, that you are not trying to open these 'excell' files on the server. (If you are, what with? Excel doesn't run on Linux. Wine??? On the server? OO?)
The out of memory error occurs on the server and not the client.
You comment on reducing the size of the data. I think that you mean the number of data elements and not the size of an individual data element, but I'm not sure.
If I've guessed any of this right, then:
-You need to say how you are serving these files. I'm guessing samba, but you should confirm this and say what version, just in case its relevant and whether there are any error messages from the file server program.
-I see no real evidence that caching is in any way involved. It may be an 'out of virtual memory' problem. It may be useful to say:
-how much physical memory you have
-ditto, swap space
and post the output from "vmstat 10 10" to give an idea of what you are seeing going on.
Also, please try to post the actual error message.