What do you define as a server? a linux box was popular. You can run YDL, not sure of current versions (gave up on Suse on Mac).
What do you want to run on it? NAS?
Have alook at Atom-based (Intel) boxes? HP has a nice unit with 5-10TB of storage, eSATA and USB as well as internal drives, just needs a good giga switch.
For what it costs to run a G5, well, they do make nice room heaters in the winter, I don't think turning it into a server is always the best, and you are probably looking at spending on internal or external storage in most cases unless you have a storage array already.
Donate. There are lots of places in need that would be happy to have one, schools, charity organizations...
Windows Home Server SP2 $99 - even if you don't go for it, the WHS Bible or manual show how to get the most out of it (comes with many of your NAS boxes). Supports Macs, PC, iTunes, TimeMachine and more.
I'm sure there must be similar $499 devices or should.
This question comes up about once a week lately.