Dude, you're giving him a false hope right now. There's a difference between being able to run the game @ 1080p/Low and not being able to run it at all. In second case, OCing is useless.
OP, If you really think that you can double the performance of your 7670M by adding few megahertz to Core Clock value, fine, go for it. Download the latest MSI Afterburner Beta, add 25 MHz to "Core" value, run the game, repeat 'till your screen freezes or you catch a BSOD (it's absolutely safe, the thermal output goes up only slightly and cannot harm anything). You can do the same thing to memory (you should probably just add 100 MHz once and then stop). For example, if you started with 600 MHz @ Core & 1800/900 MHz @ Memory, you should be able to get approximately 700 MHz @ Core and 2000/1000 MHz @ Memory. That's it. Your few percents of boost. Again, not much point, ARMA 3 is super heavy in terms of GPU utilization. If 7670M wasn't enough, it will never be no matter what you do.