If you already have an Ethernet shield that you could continue to use for this project, then buying a rather expensive WiFi shield is unnecessary.
You don't need a WiFi shield to do WiFi with an Arduino board. You can simply buy a little pocket router for $30, for example a TP-Link wr702n and connect it with the short Ethernet cable that comes with it to the Ethernet card and you're in business.
I've done this in two projects already and it works like a charm!