Engine size all depends on the other characteristics of the bike. Gearing, weight, rpm, etc.
RC-51s are a sportbike, but they have a v-twin engine. It is more torquey than screamy. A completely different beast from other inline liter bikes.
Suzuki SV bikes are also v-twins. Personally out of that line, my favorite is the SV650. It is a blast to ride... very nimble, torquey, and a light front end. It looks good too.
Interceptors are very nice bikes if you go with a newer one. I think the older ones are turds though. The newest model has a form of vtec (yay), linked abs brakes, and many other bells and whistles.
I would toss Interceptors and SV bikes into the "sport touring" category. If I had to make a generalization, these guys are the true street enthusiasts. You see them riding in full rain gear in a storm, carving corners. You see them with their cargo bags and chatter boxes riding in groups through the mountains on their way to alaska. Very agile bikes, they could hold their own on race tracks or ride beside a goldwing on a long ass trip.
I dont want to step on toes, nor am I an expert. (most of all I dont really know you ), but if you step up to a bigger bike (gsxr-750) and are not really ready for it, IMO your learning curve will flatten out and you start just trying to keep the thing tamed.
If you were not riding the hell out of that 500, tossing it around and feeling confident (not over), a 750 sportbike could bite you quick. Personally I ride a 600, and I have now for 6 years. I am not ready to have anything bigger. When I feel I have mastered my bike, and can ride it at its limits I will step up. There's nothing like embarrassing squids on liter bikes with my 600. graemlins/thumb.gif
Look hard at those sport touring bikes. Bottom line, try to ride something similar to what you end up getting before you buy it. (more than in a parking lot)