Buy new MacBook or upgrade current one?

OK so I've been deliberating this for a while now. I'm almost dead-set on upgrading my MacBook Pro Early 2011 model to either a MacBook Air or rMacBook. The issue I face with my current laptop is that it's insanely, stupidly slow. The HD is slow as molasses and the 4GB RAM just isn't enough even for a few tabs on Safari.

Upgrading to a MacBook Air 13-inch is a good $500 expense (base model) after selling my current laptop. Is that wise or should I upgrade the HD in my current one to SSD and upgrade RAM to 8GB? I can save a lot of money like this and upgrade my iPhone 5 to 5S. The SSD will be Sata 2 only compared to PCI-E on Air; is that a huge difference under normal use like browsing, iWork, movies etc etc?