I think an amazing setup would be 2 airs, over a Time Capsule (Airport Extreme + Backup Drive) to the Dell.
Auto backup your macs on time machine with the Capsule. If there is an error, its as easy at poping in the OSX disc (....may be a problem on an air =P. Im sure they have a solution for it) and selecting the Capsule.
I may propose this to mike to see what he thinks for the next computer upgrade we do.
Because thinking right now, i can imagine a single thing that could not be done, faster, on OSX that we do right now.
If your after speed (i know i am) maybe read some of these:
http://www.tweakhound.com/xp/xptweaks/supertweaks1.htm
http://www.onecomputerguy.com/windowsxp_tips.htm
Just a few "Make XP go way faster" links.
And as for clock speed - its just a marketing tool. There is some truth to it...
But I mean... a 4ghz P4 with Hyper Threading wouldn't touch a Core2Duo at 1.8Ghz.