I just configured a few different PowerMacs and Xserves for comparison purposes. To me, the Xserve looks like a better value for a graphics professional or even a geek.
For around $3200 you get 1Ghz G4 Xserve, 256MB DDR, 60GB ATA/100 w/ 3 other independent ATA/100 channels, ATI Radeon 8500, 3 firewire ports, Ultra160 SCSI. A 933Mhz PowerMac without Ultra160 or ATI Radeon 8500, less firewire ports (and none on the front!), non-DDR RAM, non-independent ATA/100 channels, but otherwise similar runs around $2500.
Of course, for the Xserve, you would need to add an external FW CD-RW or DVD burner. But all other things equal, I think I would rather have a rack-mount neatly under the desk than the bulky G4 tower. The burner could sit on the desktop, and the Ultra160 RAID could sit anywhere up to 12m away. On top of that, you get all the mission-critical features of OS X server, redundant cooling and power, and the like.
Since recent benchmarks have shown the dual 1Ghz to have little real additional value for non-video graphics work, the single 1Ghz Xserve with DDR RAM should theoretically have the edge over the dual 1Ghz PowerMacs.
If I had 3 grand laying around, I think I'd go the Xserve route. With the OS and applications on the ATA/100 drive, and the scratch/editing/storage on an external U160 RAID, it would really make a super graphics platform.
What do you think?
--Chris
PowerMac 7500/604e 200Mhz
18 GB U2W SCSI
256MB RAM
OS 8.6