Hi everybody,
I am an avid sim-racer, and on my favourite sim, Race 07 I had a nasty issue with the game crashing (and also having texture flashing problems) when using a certain track (Nordschleife - about 20 km, with a lot of trees around) with the maximum number of opponents (25). After a while of looking into the problem (changing video drivers, google-ing stuff,etc.) I found a solution that I think might help a lot with Arma 2, and I thought might be useful for you too look into:
1. I patched the .exe of the game with this tool:http://www.ntcore.com/4gb_patch.php. It's safe and automaticly creates a backup of the original (also comes with an interface so very easy to use);
2. I also enabled PAE as shown here: http://windows7themes.net/how-to-enable-pae-in-windows-7.html;
I also used: "BCDEdit /set increaseuserva 3072", as described here: http://www.mathworks.com/help/techdoc/matlab_prog/brh72ex-49.html
Fact is, after these two steps I never had the crash problem in >240 hours of gameplay. I also tested with the unmodded version of the game exe and it crashed pretty much every time or had flashing texture problems. It is also very convenient as there the switches in step 2 can be easily disabled if you don't find them useful.
I don't know if this will work with Arma 2 (I am going to try it with the windows memory allocator), but it could be useful, also for other memory intensive apps (especially games).
Hope this helps.