PowerMac G5 going for tempting prices on ebay
Eleanor Spenceley Mac Mini's G4 have been stubbornly (stupidly?) expensive selling for around £200-£250, these are now going for £90-130.
However, what is more surprising are the PowerMac G5s, you can get a decent spec'd dual processor machine for around £150.
I've picked up a beautiful 1.8 dual core G5 for under £129 last month. These are fantastic systems, the build quality is just outstanding. Mine is a second generation G5, so the processors are cooler and therefore quieter, and orders of magnitude quieter than a G4 mirror door machine.
Even with 'just' 2.5GB, this machine is so responsive for everyday tasks, I hardly notice the difference between it and a Intel Core 2 duo laptop, except for really processor intensive tasks of course. Though the G5 will run your old and still very useful PowerPC software at pretty much top speed. With reports of PowerPC support (Rosetta) going away in Lion, this will be a good machine to run your old software. More powerful G5s are also able take 8GB of memory so there will be no problems there, though with my 'low-end' G5, I have to settle for just 4GB if I ever decide to upgrade it. (Tip: Read the manual before installing memory in a G5)
If you are looking for a second hand Mac and not worried about being on 'trailing edge' of technology, I'd suggest having another look at the PowerMac G5.