As for the pricing on the iMac, you might want to factor in OSX and iLife '09. Although they come bundled with all Mac purchases, it will cost you $99-$129 each. Microsoft Vista doesn't come standard in every purchase, you have to pay extra money for Ultimate, Business, etc. with no bundled software.
Don't forget all the space you save from using an All-In-One iMac, versus the Desktop that Dell have to offer.
If you factor in everything, you only pay about 10-15% more in premium. Sometimes, you gotta look into the bigger picture and not just the hardwares.