I'll admit I only skimmed this so forgive me if this has already been said but how do you know it's an fps drop and not a ping drop? Also, when do you experience the lag? Is it when there are 20+ people on the screen in the middle of a fate?

I have a gen 1 i3, a gtx 570, and 12 GB of RAM with no fps drop (at least, none that I can perceive.) This is on max settings mind you. Sounds to me like bandwidth issues. But without more info, I couldn't say for sure.