Upgrade Mid-2012 MBP to a new 13" Macbook Air?

Hi all - I currently have a mid-2012 MBP (i5, 4GB, 500 GB HDD). I was looking at seeing whether or not the latest model 13" MBA would be worth a purchase.

I purchased this MBP for $450 and trading it in at Best Buy will get me $420. I was planning on spending extra to replace the battery, get an SSD and upgrade the RAM to either 8 or 16. But with the cost to purchase all of those parts, is it worth it for me to just trade it in for $420 and use the remaining to get a new MBA at $750 ($725 with the Visa checkout deal)?

I don't mind the weight, while it's not ideal it's not that big of a deal breaker but the weight of the MBA is a nice thing to have.

I should also mention that I'm only using it for word processing, internet browsing, watching videos and maybe some light photo and video editing. Most of intensive stuff is done on a custom built gaming PC.