Many people have commented that they see noticeable improvement when upgrading their computer's memory from 128MB of RAM to 256MB.
In the past BIS used to say that more than 256MB will not be used by OFP itself, though it might improve overall performance if you are running other applications concurrently with OFP.
However, in February 2002, Ondrej Spanel, BIS' Lead Programmer, had the following to say regarding the potential benefits of having more than 256MB of RAM:
OFP can use more than 256 MB, as its internal heap can grow up to 512 MB, if necessary, and it can also allocate some more memory out of the heap (like Direct3D resources or memory mapped files). Some memory (actually sometimes a lot) is also used by operating system and its services.
In real life, more than 256 MB is probably almost never used. If an OFP mission would require that much memory, even the most powerful CPU available today would be bottleneck.
This will probably change when faster CPUs will be available. If we consider 512 MB allocated on internal OFP heap, about 100 MB for file access and about 100 MB for operating system, we have about 700 MB used - and that is likely the theoretical limit OFP can really use.
Installing more that 1 GB will therefore not be of benefit to OFP, and installing more that 512 MB will probably have very little effect unless you also have a very fast CPU and you are playing very complex mission. Good way to check the actual amount of memory used by any application is to open WinNT Task Manager and check MEM USAGE field, which the shows current working set size of any process.
Ondrej Spanel, Lead Programmer
BTW, if you've got Windows 98 or ME, you should be aware that having more than 512MB of RAM can actually cause rather than solve memory problems. See this Microsoft Knowledgebase article for details and solutions.