Contrary I'd sell now if I can still get $1000 for it on eBay.
The prices and functionality of the old G5 can only go down as time goes on, and right now there are actually people who still need PPC macs. Further down the road the price might just bottom out on these as Intel macs become more common and start appearing alongside the older PPC variants on eBay.
Put it this way, you can sell it now and for $299 (or $100+ under EDU pricing) get the latest and greatest. Why not?
Edit: I believe that with the Intel switch, the myth of "Macs holding their resale value" will finally end.