Suleman,
1. "Can most firewalls protect against scams, hijack attempts, and which?"
No, firewalls do not protect agains scams, hijack attempts, or most viruses. Firewalls will notify/protect your computer against unwanted "intrusions" from the outside and from unwanted programs current installed on your computer from "phoning home", but because the scams and hijacks are items that the user indirectly "allows" the computer to run, the firewall won't protect against that. Because you MUST give your browser and ISP permission to access the internet, you provide the "hole" where many hijack attempts can take place. Secure the browser you are using, use caution in which websites you visit, and secure your e-mail program and you will eliminate a major portion of hijack attempts and scans.
2. "say if someone was to use internet banking, or bidding on sites(auctions) which would be the best firewall to use."
The best firewall to use is up to you. They ALL use resources and make internet connections a little more challenging. Most importantly, some computer configurations work better with one versus another. Read the information David posted above, read reviews, hardware firewall vs. software, run some tests, and see which one works for you. Most importantly, if you're running Windows 98 on a dial-up connection, do you really need one? If using an broadband connection, do you use a router, and does it already have a NAT firewall istalled. Only you can decide whether you need it.
Hope this helps.
Grif