Just ride what you got now, the fx is a pretty versatile bike and maybe relatively slow, but if you get some fitness into your legs you should be able to maintain 17-18 on it...especially if you put some clip on aero bars on it. (that's what I did when I had an fx)
changing tires is a good idea too. I suggest Compass Tires, something like the stampede pass or even
Grand Bois Cypres. You will probably feel a difference in rolling resistance compared with your current tires.
Compass Bicycles: 700C Tires
+1. I have the Cypres Extra Legers on my cross bike and they easily ride faster than mid-range 700x23s. Fat tires have a reputation for being slow because most of them are geared towards commuters and have stiff casings and thick treads. High quality fat tires ride just as fast as skinny tires (until you get to the point that tire aerodynamics starts to matter, but that's much higher than the speeds we're talking about here).