My best friend is in a similar position as you, just starting to dip his toes into fly fishing this year, with a 7'6" 3/4wt that's been sitting in his attic for over a decade.
The (very) generalized guideline I gave him for deciding what water his rod would work on was this:
1) Wet water. Seriously, you can divide any creek into small sections and fish it effectively with just about any rod. If you want to fish a creek, fish it. Any rod will work better than no rod.
2) The 4-6wt range work great on any trout water in PA. This is kind of the do-it-all range.
3) 6-8wt is for warmwater (not for the fish but because the flies are generally bigger and heavier) and steelhead in Erie.
4) As a (very) general guide on when to go below a 4wt, I suggested that in any stream where, in the entire section he's fishing, he knows that he can walk across anywhere without going in over his waders, that's a prime candidate for dropping below a 4wt. Generally around here, the bigger water has some deeper holes and pools and heavier curents & rapids than you'd really want to wade through, choosing a better crossing point. If you could pretty much cross anywhere if you had to, it's likely perfect 3wt water. If you can jump across at several points, you can probably safely drop below a 2wt.