I am replacing an aging old crappy TV that barely turns on now. I haven't been looking at the TV market for some time.

Are there certain brands better than others?

Which is better for gaming, Plasma or LCD?

Will SD channels display well on a HDTV?

Could someone point me in the direction of a good model? 720p is what I am looking at and my philosophy is as follows: The cheaper, the better. 600 dollars is the maximum, I'm really looking at 500-550

LCD is better for gaming.
LG, Samsung, and Sony are the only brands I would buy from.
Yes, SD channels will look fine on your TV.
I suggest 1080p since your TV is going to be over 40", but it is your call.

Click Here for a damn good LG TV that fits your info.
Click Here for a damn good Sony TV that is 1080p and cost a little more.

If you need anymore help, just let me know.