Any wheel on any fork / frame?
When buying new wheels, make sure the spacing matches up. Road wheels typicall use 130mm rear and 100mm front spacing where as most mountain bikes use 135mm rear and 100mm front. Hybrids vary, but it looks like this one has Shimano RM30 hubs which are 135mm spacing like a mountain bike.  However, if you find a 130mm spaced wheel you like, you can add spacers to make it fit the 135mm spacing of your bike if need be. 

I think it's worth a shot trying out a 700x28c tire on this bike without changing the wheels, which would be much cheaper than switching the wheelset.

FYI, I got my spec information for the wheels from here: http://www.firefoxbikes.com/hybrid_Trek_73FX.html