In my albeit biased opinion I would recommend

FAR CRY

That is If you haven't tried it yet, excellent ai, graphics and gameplay. The only downside is the low quality voice acting and the lack of a coherent plotline.

And the icing on the cake is that by the time you will be done with it, it's successor crysis is due to be released in the the third quarter of this year.

I would even recomend the the orignal precursor to ArmA, the Operation Flashpoint series.