Why go for a 660 ti when a 760 beats it? Hell, you could even get a 7950. They're all around the same price.
OP, L4D2 won't take much to run (I could run it on an Athlon II x2 and a 6670), but I'm fairly sure Arma II will need a good CPU, as it's heavily CPU based along with a good GPU. I don't know what to say, but probably no less than a 7770.