What are my hardware upgrade options for a late 2009 iMac?
Whether it's 'worth it' will be up to you.

You can get 16GB RAM in there &amp; you can swap the HD to an SSD. 

The SSD will feel like day after night compared to an HD - especially as you're on an OS that is designed for SSD &amp; almost forgets spinning rust ever played a part. 

You will be bound to 10.13 so you might not need more RAM  unless you’re sure you will run apps where the current amount is not enough. 

Since SSD is a big speedup, more RAM will let it 'breathe' a lot better doing multiple tasks... like having more browser windows open. And you’ll likely want to use it more once it’s speedy. 

It won't turn it into any kind of powerhouse compared to a 2015 MacBook even or any new portables, but inexpensive upgrades will make it all feel more relaxed. 

Expect 4 to 8x speedup in system start time with an SSD that’s moderately priced.