The main difference usually pointed out between the swinger and poly lifestyles is that swinging is about the sex and poly is about the relationship. If your girlfriend wants sex without any emotional connection she is somewhere toward to swinging end of the spectrum.
I think it makes sense to have a friend with benefits arrangement rather than going out to find someone new every week. The friend is a known quantity; they've already passed the "possible axe murderer?" test, you know what they're like in bed, and you know where to find them when you want them. Personally I also like the emotional connection that's not there with someone I've never met before, but that's my preference.