Well, one year ago, I replaced my white Macbook for a HDD Rev. A and 24" iMac. The idea was to use the MBA as a secondary, mobile solution, and the iMac as the main computer.
I had to wait 3 months to get the iMac delivered by sea, and because of that, I've used the MBA as my sole computer for a long time.
When the iMac came, I was really happy about it, but after a couple of months, I've started to notice that I was only using it for leisure, specially because the MBA is so portable that I've started to use it at work, replacing the HP laptop my company gives to me.
So, I still have the iMac as a "main" computer (where I download and store media), but I rarely use it. And I'm hooked up to my MBA like 8 to 10 hrs a day.
I plan to sell them both and get the best MBA money can buy as soon as it refreshes.
I also think about buying a MBP with SSD, but I think the portability of the MBA is unmatched by the biggest screen on the 15" MBP.
CPU, Ram, HD size, it doesn't matter that much, since OS X is tidy and I don't mind using an external HDD for non vital media files.