The router has a firewall and some other security features, but you still need the big three: firewall, antivirus, and antispyware for each computer.
Keep that router; it's a good one. You don't need more hardware. Just keep the firmware up to date and employ the wireless security. For that matter, keep your operating system and security software up to date too.
I recommend the following:
ZoneAlarm firewall (free)
AVG antivirus (free)
Microsoft Antispyware Beta (free)
Spybot antispyware (free)
Ad-Aware antispyware (free)
Use only one software firewall per computer to avoid headaches. I have Windows XP's built in firewall turned off and ZoneAlarm turned on. Notice that there is more than one antispyware program. They are all pretty good, but redundancy keeps your computer safer. What one program may miss, the other may find.
You will get recommendations for other utilities that are very helpful, too.
I'm trying to setup a wireless LAN. I have a Linksys Wireless-G Broadband Router-WRT54GS, v. 2. I'm being told that I need to install firewall hardware rather than software to have real protection. First, is that true? Second, what kind of hardware would I get and is it in addition to the Router I have or am I supposed to replace the Router? I'm working with a Dell Dimension 300 and Windows XP.