I would go with the OCZ 2 Vertex 2 and hold off on adding additional 4GB of RAM. You'll get more benefit with your system overall by adding a SSD. The 120GB gives you the room to work more comfortably with your system. Depending on your usage, 4GB really won't add much of a benefit to the overall system.
I was over at the Xbit Laboratories web site checking for new information about ssd's. I found a very interesting article published last November containing facts and forecasts about Flash and Solid State Drives. There is some good news and some bad news.