There are several sites you can visit which will allow you to determine how vulnerable you are to hacking. In essence, each of these sites attempts (with your authorization) to access your computer. The extent to which they are successful (or not) allows you to gauge how vulnerable you might be. Some of the sites that I visit at irregular intervals are:
www.grc.com (ShieldsUp option -- firewall test. Also has LeakTest software to see if info can be communicated *from* your PC to the internet)
http://www.auditmypc.com (firewall and other tests)
http://scan.sygate.com (firewall, trojan tests, etc.)
The best you can hope for in these tests is "full stealth" -- in other words, a hacker attempting to access your computer would receive no indication that there was an active connection at tyour assigned IP address.
Bear in mind that you really need to worry about bidirectional security. For instance -- most folks only worry about hackers trying to get in. It's just as important to know if any of your installed software is trying to send information out without your knowledge (spyware, perhaps). For this reason, I not only run a NAT box to isolate myself from hackers, but I also run ZoneAlarm which will detect any attempts to "phone home".
A good introductory article on this topic may be found here:
http://www.dslreports.com/faq/9787
Whatever you decide to do, make certain that the firewall hardware/software is compatible with any trading software you may wish to use. For instance, if you use a VPN connection, make certain that the hardware/software you use works with VPN standards (commonly something like IPSEC). I'm not terribly familiar with VPN standards, so I can't comment on this intelligently other than to note that it can be an issue for the unwary.