Will another video card help?

Hello,

I upgraded my computer over a year ago, and have been extremely happy with it until I upgraded my monitor. My monitor now uses 1920x1080 resolution, and I'm beginning to wonder if my processor is a bit too slow, or if buying another video card would solve some of my issues.

I'm very picky about frame rates. If I'm not getting constant 50fps+ while playing a game and I dip down to around the 15-30fps area, it is a huge annoyance for me.

Current specs:
E8500
1x EVGA GTX275 896mb
6gb DDR2 800
EVGA 750i SLi
Corsair 850TX

My question is this: I was thinking about buying a second GTX275 to hopefully help with the frame rate issues. Will this be a large improvement, or is my CPU currently a limiting factor?

Will appreciate any help I can get.  

Thanks.