First, there aren't really any BAD seats in Autzen, it's one of the advantages of a smaller stadium. Everyone is pretty close to the field. Second, as you can imagine, the visiting team gets the worst seats in the house. In Autzen those are the ones directly under or near the "BIG" Screen.
Getting two tickets for $300, you're probably going to be end zone or corners of the end zone at best. The closer you are to mid field the better. Best seats for that price might be section 26 (probably the best of the corner sections) or the or 15. 35 or 36 would be good as well and put you closer to the majority of Tenn fans. Section 5 or 6 might be a little "rough" as it's next to the student section.
$500 will get you some pretty nice seats if you want "very good seats".
As for the visitors section I'd rather be in the lower rows, so you can turn around and see the Big screen easier. Again, there are NO bad seats in Autzen. At least that's my opinion.