Well, here's the best I can do... compare the followign system to a 17" 1.8ghz iMac ($1500 new, $1400 edu, $1300 refurb)...
-2.8ghz P4 (this is about right speedwise - don't compare a G5 to a Celeron!)
-XP Pro (OS X has all the features of pro... only fair right?)
-256MB RAM (you'll want more in either system)
-80GB HD (the iMac has a SATA which will probably be a little faster)
-8x DVD-RW (Superdirve)
-RecordNow and MyDVD Delux (looks like a psuedo iDVD type app... gotta have something to use w/ DVD-RW)
-Intel Integrated graphics card (the iMac's 5200ultra may suck, but it destroys this integrated, shared memory piece of crap... on an aside, anyone know if these systems even have AGP ports? Dell doesn't offer any upgraded video card options for their lower end machines, and I can't find a listing of available expansion ports on their site).
-17" LCD (which won't compare to the iMacs display... I use a Dell 17" LCD at work and it's nice, but the iMacs display is gorgeous and widescreen!)
-Firewire add-on card (built into all Macs, essential for anyone wantign to do DV work)
-Basic Speakers (iMac has some decent built in sound... about equal to basic desktop speakers, imo)
-Keyboard + Optical USB mouse (APple Pro Keyboard is very nice, imo, and the mouse is usable

)
-MS Works (Appleworks comes with iMac)
-McAffee virus scanner, internet firewall app (Mac OS X will still be more secure than a PC with this software, but you HAVE to do SOMETHING with a PC)
-1yr warranty (same as basic Apple warranty)
Total: $1192 (current 10% off sale brings it down to $1,084)
So a little bit more for the iMac, but you also have to look at what 'extras' you get with the Mac...
-all in one design is great for saving space and just looking good
-VERY nice display as opposed to adequate (1440x900 vs. 1280x1024)
-better video card in case he (or kids?) wants to do any gaming down the road (sad when the iMac has a better video card than a competitor)
-iLife apps... the only thing we added to the Dell was an iDVD "equivalent", there's also iMovie (great if he wants to start getting into DV!), iPhoto, and Garageband.
-better customer service... Apple's isn't perfect, but it is leaps and bounds better than Dell's... at least AppleCare reps speak english, and can (usually) do more than read from a computer prompter.
-and as mentioned earlier in two years that Dell will be worth about... $250, and the iMac G5 will still get $800-900 (assuming the used PC and Mac markets stay how they are now).
-lastly, if he has a family, OS X's multi user account system is infinately better than Windows XP. There's really no comparrison there (just like most of Windows vs. OSX

)
Anyways, that's the best I can do for comparring a Dell to a Mac... try and get him out of the 'megahertz marketing myth' and take him to see an iMac in person... they are amazingly impressive when you are standing in front of one... see one Dell, you've seen them all, but the iMac isn't even in the same ballpark as a regualr desktop PC... not even the same game!
Rob