Glenn,
The voice lessons will help quite a bit with breath control and expanding your range--at least it did for most of the folks I know, including myself. However, if you're singing results in a sore throat, you're doing something wrong. I would suspect that you're either singing a note or two higher than you should be, or trying too hard to project your voice.

Only perform songs that don't cause you to strain your vocal-folds (cords), learn to use a mic effectively and practice as much as possible. Two hours a day is just about right for practice, but if you haven't been performing for sometime, begin with just one hour a day, then gradually increase it to two and then three or more. It's a lot of work, but it pays off in the end.

As Donny said, record a couple midi files, then sing along with them, and if possible, record the performance on a tape recorder or CD. Then listen to your performances and be objective. You'll quickly hear the mistakes, and with additional practice, you'll be able to correct them.

As for the lessons, some instructors cannot play a song in any key other than the one it was written in. If this is the case, this could be the root problem with the sore throat. If the song seems to make you strain, have them play it in a lower note where it's more comfortable. That's why they put transpose buttons on keyboards.

Final thought. When you get a sore throat, one of the things that soothes your throat instantly is a dill or sour pickle or onion. Years ago, when I was a newscaster for a local radio station, I learned this little trick and it saved my bacon dozens of times.

Good Luck,

Gary