If I wanted a computer to last ten years I would go for specifications that actually exceed today's midrange to high end machines. Say... 16 GB of RAM, instead of 1.5 GB, and you'll probably be okay. And instead of 256 MB of VRAM, 2 GB might be better. It's really hard to predict considering how many things change. I mean, if you were to somehow give an old Power Macintosh 8100 a 1 GHz processor and 1 GB of RAM, it would be fast, but it'd still have ADB, serial and SCSI ports instead of USB and Firewire.
And who are you to talk, Messiahtosh? You're the one who is selling your PowerBook and buying a new iMac just because you think it'll be so much faster. Is it because you think the G4 sucks and the G5 is so awesome it'll last ten years and still feel fast?
Personally, my dream machine would be either a thin, widescreen laptop that slides into a dock for use at home with a larger monitor (a la Duo), or an ATX-compatible motherboard for which I can purchase a case, drives, a processor, and so on. It would be so cool to be able to go on NewEgg and put together a Mac-compatible system from off-the-shelf parts. As it is now, the motherboard, power supply, and processor are all special and must be obtained separately. If I could just put together a $400 system with an Athlon XP 2400+, Radeon 9600 Pro, and a basic nForce2 motherboard, and run the MacOS on it, I'd be in heaven.