Upgrading from a Nikon D50 - any suggestions?

Hey everyone, I just recently started shooting with a semi-serious camera and I've been having a blast. In the last month I've taken more pictures than I probably did all of last year.

I'm currently using a Nikon D50, which is a hand-me-down from my mom and didn't cost me anything. I've gotten pretty comfortable with the camera and enjoy using it, but I feel a little limited. I'm getting some really great images out of the D50 and know that it still works great, but with all that said it is almost a decade old and I think I would have a better experience/more fun with a newer camera.

I'm a long ways from doing anything professional and don't have a professional budget so I'm open to semi-older cameras. I'm looking to stick to as close as $500 as possible and would prefer to buy refurbished. As a side note selling the D50 is not an option as it is basically on extended rent for free from my parents.

The camera that I'm looking at the most is the Nikon D5100. I like the articulating screen and the iso performance is not bad at all (and works better than a D3200 - which is one of the other cameras I was looking into). I like Nikon and have grown up with them so I'm inclined to continue using them, but I have nothing invested in DSLR gear so I would be open to using another system - although it would take quite the feature set and image quality would have to be on par or better.

What are your thoughts on this? If you liked Nikon and had ~$400-$500 for an entry-mid level DSLR w/lens (kit lens would be fine for now) what would you go with? I want a solid body that I won't have to replace in a year or so - I'd much rather be putting that money into new lenses.

Thanks in advance!