I'm kind of in the same position as you and I'm ordering the 1.8 Powermac right now actually. If you're looking between the iMac and PM, I'm guessing you've already got another display. Pricing is identical between the two and you have a much better upgrade path with the PM regardless of the fact that all PM's will be dual cpu or dual core soon. You'll be buying an "out of date" system regardless of what machine you decide on.
Also, Just a heads up though, if your Dell actually is old as you put it, the HD likely won't swap over into the powermac due to it not being SATA.