Well, i no longer own a GZ (kinda started this when i did, then moved up lol).
I spent about 3 weeks trying to decide on what to get. I was originally going to stay with suzuki until i sat on an Intruder (yuck). They were very 'tight' fitting, although they dont look it looking at one. I browsed pictures of many many bikes on ebay, google etc. I compared prices, price ranges, reviews... you name it i went through it. I then headed on over to the dealer and sat on various makes and models. I FINALLY decided on a Honda Shadow, forget if it was an ace or comparable, it felt nice and fit me very well. What i ended up getting was a Honda Shadow Spirit 750. (basically the same bike, smaller fenders). It wasnt what i decided on exactly, but i got a really good deal on ebay. Paid blue book for it but it also had almost 2 grand in work done to it (cobra pipes, cobra jet kit, mustang seat, hypercharger, new paint). I just couldnt pass that up. The spirit still fits me as good as what i sat on at the dealer and is plenty of power for doing 100 mile runs just riding around on back roads and on the highway. The stock seat sucks (i got a stock one just to 'have' one). I took it off after 15 minutes of riding and put the mustang touring seat back on. All in all, the 750cc is more than enough power for what you want to do.
What you NEED to do is go to the dealer and talk to them. Tell them you want to buy a 'new' bike (just lie to them lol, they bend over backwards to help you out). Sit on all the bikes that interest you. Sit as long as you can on each one. Stand it up to feel the weight and balance. Sit on it and put your feet up on the pegs. Get a good feel for each bike. You should be able to narrow it down to 2 or 3 ... maybe even 1. I am sure you are looking to buy used, trust me, thats the best thing to do until say your NEXT bike. So now that you have narrowed it down..... go find yourself a really good deal on a used one. Theres plenty on ebay with saddlebags, windshields already installed. You can find some good deals on there.. as well as some overpriced ones too lol. So do your homework on bluebook values!
Most dealers will tell you, as soon as most of their customers get their bikes home, they buy a mustang seat for it for long rides. They are not lying to you! If you plan on getting one, plan on getting an aftermarket seat! Unless you buy a VStar or something comparible with those comfy pillow seats
This is where 'used' can save you some money. Alot of used bikes come with aftermarket seats
As for windshield, you will be looking at $160 and up for these type of cruisers. Saddlebags you will want to spend a little more and get something name brand and heavy weight leather, trust me. Heated grips, you can find on ebay for $50 shipped.
Performance and handling. Most all these bikes handle about the same (minus the minibike handling of the GZ). My spirit handles the road great. Its sure no sportbike but i can rip throught twisties very well. They take bumps MUCH better than the GZ does.
I will say though, whatever you do decide... it will feel like a monster when you first straddle it. The ride home you will shit a brick at the power increase, ride comfort, handling, smoothness of shifting etc ... You will also have a smile on your face a mile wide and think to yourself the entire ride home "why the hell didnt i do this sooner". (this is all from personal experience too lol).
If you are very confident on the GZ, and i mean to the point that you can whip that thing around like a minibike when you were 14.... You will have NO problem going to a 750cc and could possible even up it to an 1100 if you REALLY wanted to. The weight difference is like 20 lbs or so depending what model and really, the only diff is torque. 1100 will get you to 60 faster than a 750 would. After a few weeks on my spirit 750 i immedialty began to think i could have very easily went to an 1100 (which will be my next bike if and when i finally sell the spirit 750).
Good luck on this, i found looking for an upgrade from the GZ a real pain in the ass because of SO many choices. But if you follow what i have said (and any other forum you ask this question would tell you to do the same thing) you should have no problem finding yourself a good motorcycle that you are comfortable on.
Any other questions, fire away!