I got the 500 to boot off a iMac install CD (my iBook only came with 10.2 install CDs) so it seems in ok shape apart from the hard disk.

As for the 800, I read about applying heat to the chip to get it to re-solder itself (either with a heatgun or this). Then there's the shim method. I guess the heat method would be the more long-lasting, but riskier way.