I have an i5 2400 and ArmA2 is one of my favourite games - in fact, I run the weekly ArmA2 event here on SimHQ.
I ran E08 Benchmark from ArmA2 OA. I use a resolution of 1680x1050 with visibility at 1600 and all settings on 'very high,' except for anti-aliasing, post process effects, and HDR quality which are set to 'normal.'
Testing with fraps, my frame rate maxed out at 60fps viewing units in open areas. Viewing unpopulated urban centres, it dropped down to around 45fps. Watching infantry and tanks rolling through the same urban centres, I got around 30fps. The benchmark reported my average fps as 36.
I only have the ROF Demo, so I loaded up the Observation Escort mission. Playing with absolutely all settings at maximum yielded a fairly steady 25-30fps, flying at tree top level with about a half-dozen planes withing visual distance. A bit lower than I care for, so I could probably dial back the AA from 8x to maybe 2x for a boost.
Overall I love my i5 2400 and can't recommend it enough. Great bang-for-buck performance. I can only imagine the 2500k is better.
"...for who are so free as the sons of the waves?"