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The key question is how large your DEMs are. It sounds like they are huge, so that the amount of data involved is very much larger than the RAM available, which means your process is paging out to disk all the time and thus is disk bound. Unless you can add enough RAM to avoid having to page out to disk (unlikely), adding RAM will not speed up the task a whole lot since it is disk-bound, not RAM bound. Having said all that, well, adding RAM is almost always worth it because it will probably give a small speed increase anyway and, let's face it, RAM is cheap. If it was me I'd put in the max RAM the system can handle (probably 3GB total). Just like you can never be too thin or too rich, you can't ever have too much RAM. :-) And, there is a chance that your process will fall under the magic limit where everything fits into RAM and then it will go significantly faster than now. Another approach is to increase the speed of your disk subsystem. There are several strategies here, such as mounting your TEMP folder on a large, fast drive, or perhaps using a RAID system with multiple hard disks configured into a single striped volume. Such techniques can easily increase your disk subsystem performance by a factor of four.
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