I'm currently playing Dayz Arma II mod, on my laptop (Lenovo G770, i5 2.5GHz, 8gb ram, radeon hd 6650m 2gb)
The game works fine, the only problem is the frame rate, I get an average of <20 fps sometimes as low as 10 fps.
I play MW3 and always get 30-40 fps.
Is there a good external gpu that would work with my laptop, I heard on PC Per some time ago (can't remember which episode) someone recommend an external gpu.
The best solution for your laptop would be to sell it and build a proper gaming rig or buy a real gaming laptop.
All of the external GPUs I have seen use thunderbolt, I doubt that that your laptop has a thunderbolt port. Even if it did an external GPU would probably be way too expensive.