Those are going to look horrible together if you are planning on playing on two screens at the same time.
Not only that but I find gaming on two monitors to be like, near impossible especially in FPS where your cross hair is in between the monitors. You need three monitors really.
With that said if you increase the resolution of course you are going to see a dip in FPS. A notice one too.
I'd just play single monitor on your new monitor and forget using the TV.
I wasnt going to use it for a dual monitor display while gaming, just use the web while playing a game and just having two monitors together would be pretty cool. Also how much FPS will I drop? With that drop would it be better off to just get a 1080p monitor?
Ohhh well thats different. A lot of games disable your second monitor while you are playing them though, just so you know.
I can't tell you the amount of FPS you will drop but if you go from 720p to 1600x900 or 1920x1080 then your FPS will drop a fair bit depending the game. It is harder to render an image with 2 million pixels as opposed to 1 million pixels.