Your banking site is secure. The communication from your PC to the bank is secured via SSL. That's not the part you have to worry about. Being on Wifi means that anyone else on there can access your computer. If you do this often it's a good idea to get a firewall between your computer and the internet. There are many firewalls that will let you connect to Wifi internet, but they may cost you over $500 for anything half decent. On the cheaper end of things, make sure you have a software Firewall running, and consider SNORT. (Intrusion Detection.) The link on how to install Snort is here:
http://www.youtube.com/watch?v=7Pg_ZJV4cSY. I did not include a link for a firewall since most people just go with the built in Windows Firewall.