C2D vs. Core i5

So I currently have Early 2008 model Macbook Pro 15" (MB133xx/A) with Following Specs:

C2D 2.4G
200 GB HDD

I want to upgrade to the new Macbook Air 13" with Core i5.

Just want to know if it is a significant upgrade, if not should I shell out that extra $100 for Core i7 model? (I dont game on that laptop, just use Photoshop Elements and Lightroom most of the time for my photographs and other small image editing. Other than that I am a heavy MS Office user, tweetdeck and heavy on the browser for work.

I am very much inclined to buy the MBA mainly for its portability.

If anyone is able to quantify the speeds of both the processors please let me know, as I do not know the unit of its measurement.

Also , when is the next iteration of MBA is due and should I wait for it?