If you have the space and use for it, I'd say keep it. If not, you can get a decent price for it. I'd say it'd be worth about $150-$300, depending on its condition, working state and installed software. If you bump up the RAM a bit more then you might be able to get more money out of it, if you do decide to sell it.
I think the G5s are still quite useable even in today's world. Obviously they aren't going to run the latest versions of Final Cut Pro X or Logic Pro X, but they do seem quite admirable in performance with applications designed for them. I'd like to have a Power Mac G5 quad to replace my Power Mac G4 because it'd be faster, but then again, I don't have the money at hand.