I like a lot of what Graphicsman said, but I just wanted to add my own take on things.
Problem solved...Chad Waterbury did all the hard work for you. Total Body Training:
It's not necessarily overtraining, it just might not be a type of program needed in your situation. You've been doing what you're doing for four months. If you're still seeing results, that's great, but you should plan on mixing things up soon.
Eliminate isolation exercises - If your goal is muscle growth, then I'd say bad idea. Minimize isolation exercises - I'd say it could be a good idea.
Generally, free weights win out over machines. Barbells and dumbbells require your whole body to do more work, which is almost always a good thing. The leg press is a decent alternative and a fine second place choice to squats; Rowing machines are also fine as long as they're mixed in with barbell or dumbbell rows, I'd almost say they're essential; The Smith machine is where I see the biggest problem.
If it's a matter of you not having a spotter, either ask a random dude in the gym for 20 seconds worth of help or use dumbbells. The Smith is notorious for being killer on joints. Even the "ergonomically designed" models with the "realistic 7 degree angle" or whatever it is.
The biggest problem with machines is that they tell your body where to go, instead of the other way around. If you have any kind of joint problem, this will only make it worse over time.
Last thing, I didn't see an exact question about the title of your thread...
So I'll just give you my general take on it. You need to figure out your goals first and foremost. That will let you know what volume (sets x reps) you need per workout, as well as the general reps per set. From there we can figure out the total sets needed, and then we choose the exercises that can hit that volume most efficiently.
So, the number of sets per muscle group (or per bodypart depending on your plan) is really one of the last decisions made when planning a program.