i'm not sure what the FM7 is but if you mean softsynth like VSTi's and DXi's then yes, you can play them in real time with a midi controller. The most important thing with that though is latency. Make sure your computer is pretty decent and that your soundcard supports ASIO or WDM. You want to get that latency under 5 milliseconds or you will be able to notice it.
As far as the connection, USB will work just as good as a MIDI imput. My evolution (m-audio) 61-key keyboard is awsome and it hooks up though USB. The best thing about USB is that the keyboard can get its power from it so i don't have to attach another AC power cord. I have heard strange stories about USB not working and thus people having to use the MIDI imput insted but i think i've heard that once...ever, definately not an issue 99% of the time.