There is an unusually large overlap between the PowerMac and iMac lines. The midrange iMac seems to be everything the base PowerMac is, and more. (Minus internal expandability, of course.) Update: The single-CPU PowerMac G5 is no more.
Compare the midrange ($1499) iMac and the base ($1499) PowerMac:
|Model||2 GHz 17" iMac G5||1.8 GHz PowerMac G5|
|CPU||One 2 GHz G5||One 1.8 GHz G5|
|Bus speed||667 MHz||600 MHz|
|RAM||512 MB||256 MB|
|Hard drive||160 GB||80 GB|
|Optical Drive||8X dual-layer SuperDrive||8X single-layer SuperDrive|
|Video card||128 MB ATI Radeon 9600||64 MB NVIDIA GeForce FX 5200 Ultra|
|Ports||FW400, USB 2.0||FW800, FW400, USB 2.0|
|LAN||Gigabit Ethernet||Gigabit Ethernet|
|Wireless||Airport Extreme, Bluetooth||none|
The iMac obviously lacks internal expansion, and you can't hook up the monitor of your choice*, and FireWire 800 is missing, but look at everything else: the same amount of money gets you a faster CPU, faster bus, twice as much RAM, twice as much disk space, a dual-layer DVD burner instead of single layer, and twice the VRAM. The iMac also comes with Airport Extreme and Bluetooth, which are optional on the PowerMac. And to cap it all off, the iMac comes with a built-in 17-inch LCD screen! Even without the LCD it would be the better machine for most people, and the fact that it comes with that great display on top of everything else is unbelievable.
iMac pluses: 17" LCD, Aireport, Bluetooth, dual-layer DVD burner, more RAM, more HD
PowerMac pluses: FireWire800, one free 3.5" internal bay, 3 free PCI slots
Upgrading the RAM, HD, video card, and wireless on a PowerMac to match the iMac brings the price up to $1828, and you still only have a single-layer DVD burner and no display.
Note to techies: as far as I can tell, the chart above is rather simple but everything I left off is identical--the RAM is DDR400 on both, the hard drives are both SATA, and so on.
* iMacs can run an external display, but it has to be analog and can only show the same thing the internal display is showing. PowerMacs can run two analog or digital displays.
Update: as of Thursday, June 16, Apple is no longer selling the single-CPU PowerMac G5. (Thanks to TUAW for the info.) The front page of the Apple Store now says "Power Mac, from $1999" and it is indeed gone from the main PowerMac page.