If you are like me and thought to use your Mac for productivity, games, email and web (I also do software development) then you will likely derive little or no benefit from a PowerMac Desktop.

One advantage they have is expandability and most PCI vendors don't make stuff for Mac (TV tuners for example). On the other hand the dual processor PMacs offer significant power over all other Macs.

I can only see a PowerMac being a necessary purchase for someone doing multimedia productions that require high-end PCI devices (which are, very often, made for Mac).

The ability to pick and choose the display you use is a significant benefit, but the box itself is so expensive that it limits your choices if you aren't wealthy. I suppose the Apple displays would look excessively expensive if the PMacs were priced competitively (US$800, US$1400, US$2000 respectively).

I haven't priced a PC lately, but I seriously doubt $800 will get you:

FireWire 800
802.11g Wireless-ready
Gigabit Ethernet
Plus a great case (that frankly hasn't been matched by any of the big PC makers, IMHO) -- it is not without fault though (lack of bays and PCI slots)

Plus all the other features which are pretty standard...
add to that the many extras that are expected with PPC 970s (900 MHz FSB, dual-channel DDR400, etc), and Apple wouldn't touch the $800 price point with a 10-foot pole. Stop dreaming and remember that Apple is a niche-market company with higher margins that Dell et al.