Main work computer/PC replacement - don't think so [32Mb video RAM and a puny Radeon 9200 do not cut it, especially not for my line of work (DTP)]; HOWEVER, as a competitor for mini-ITX boxes (often used for small and quiet media boxes)it's a goer - the media software is pretty much all present (though for a full Media PC you would need to add external video capture hardware and probably an external FireWire drive as 80Gb will not cut it for TV capture), it has the most important interfaces (USB2/IEEE1394/Ethernet), the graphics are faster (despite only having half the graphics frame buffer - the mini-ITX boards have 64Mb of video RAM), and the CPU is in a different league entirely (serious floating-point grunt power!)while consuming relatively modest amounts of power. If I needed a media computer, I would be tempted, but I will stick to my AMD64 Shuttle box (with its spiffing WD 'Raptor' HD)for 'real' jobs.
In reply to last post, PC emulators work pretty well considering the vast architectural differences between PowerPC and x86 processors (someone I know with a G4 733 is running Win2K SP4 on Virtual PC running under 'Panther').