The 8GB may be useful if you do multitasking, that is, you have a heap of heavy apps open. Like heaps of tabs on chrome, iphoto, word and other apps you may use. The upgrade from an i5 to an i7 I believe is completely unnecessary for a macbook air, purely because I think that the things you would generally do on a macbook (browsing the web, movies, music etc) wouldn't utilise any of the benefits the i7 CPU's offer. So it's just wasted potential really. If you were doing video editing, or music production, you'd benefit heavily by using i7's, but if you were to do CPU intensive tasks, I'd say you'd be better off with a Macbook pro in the first place.
So basically, consider the RAM upgrade, but I don't think the CPU upgrade is necessary.
With regards to the moving to portable question, I sure have!

The stuff I do on my macbook is the general stuff I used to do on my main computer, again, movies, music, that sorta stuff. Add in the fact that Mac OSX just seems faster, I had an overall better experience!
Generally speaking, with computers these days, even getting some of the cheapest ones out there is more than adequate for most peoples uses. It's kind of like cars today. You can get cars with little 1L engines and 5L v8's. But when you do general stop start driving to and from work, you won't be able to really feel the difference!
I think the best upgrade these days is an SSD over an HDD. Speed differences are absolutely insane. My macbook pro boots from touching the power button to opening chrome in 12 seconds!