I don't know why you guys are having such trouble.
I currently run an AMD Athlon 64 X2 5600+ @2.7Ghz, HD 6850 @ stock clocks and 4Gb of DDR2-1066 on a 250GB 7200 RPM 8MB Cache.
My GPU is decent(ish) but my RAM is so old and slow, my CPU is ancient and my HDD is also ancient (all but the GPU came from a PC I bought 5-8 years ago.) yet on 1080p on medium settings (I always change shadows to lowest on any game, though.) I can still record on MSI Afterburner with 20-30 FPS. Sure, turning the camera to look at a place that I haven't looked at yet makes it drop for a few seconds but that's it. I don't see why the i5 3570k @4.5Ghz can't do that in combat/action when my rig can do roughly 15-25 during World Events.
Though, one thing I noticed is when I played the game on Fullscreen I got really really low FPS readings, yet when I changed to Windowed Full Screen it went up by about 15 FPS. Suggesting this to other people never seems to help, though, so who knows why.
I've also watched videos on Youtube of people playing on the highest settings with the i5 3570k and they get amazing results.
If you aren't already, make sure you're recording to the Drive that isn't being used for the game. That might help a little. If you're recording for Youtube make sure you have your recording program set to record at 30 FPS since Youtube cuts all videos to 30 FPS. Though, since you'#re trying to record much higher I don't think it's for Youtube.
Apart from that, I don't know what else could be wrong other than the simple things like make sure Skype and Windows Live Messenger and whatever else (Google Chrome takes an annoying amount of RAM for example). You'd be surprised the amount of people who forget to close Skype or Windows Live Messenger when no one's currently talking to them.