Talking in lecture is one of my biggest pet peeves, and to be honest, you have paid to hear that lecture from any seat in the lecture hall. I posted this elsewhere but I've found the following most effective:
"Hey guys, I'm sorry, but I'm having a REALLY hard time hearing the prof, do you guys mind keeping it down? Thanks!" *smile*
I've found that this approach works really well because people generally feel bad when YOU apologize and they know that THEY are the ones doing something wrong, and it gets them to shut up. The smile helps.
In my better moods, I've used:
"Hey, I'm paying to hear Dr. Lecturer speak, not you..."
This works most of the time too, and it gets out all the aggression you had pent up before you said something. I have noticed a new breed of student who I especially can't stand, though, who just laugh and keep talking. What is this, Grade Nine? Grow up!
In all honesty though, you ARE in the right for saying something, so don't be afraid to. Sitting in front helps, but there's nothing you can do if you do go to the front and Chatty McChatterson decides to sit beside you. That's when you gotta pull out the big guns. It's your education, you deserve to get what you paid and are working hard for.