Mas typically have decent resale value, so selling both of those machines shouldn't be much of an issue. The iMac should sell pretty easily on Craigslist, for something fairly close to what you paid for it. My recommendation would be to buy the new computer, migrate over to it, and restore a clean OS onto the iMac so you can sell it.

The PowerBook G4 is probably worth at least a few hundred dollars at this point, but I can't say very precisely since you didn't specify much about it. I'd suggest going onto eBay and seeing what similarly-configured PBG4's have recently sold for. Then rebuild yours into a clean OS and sell it either to a friend, on Craigslist, or via eBay. Shouldn't be difficult.