If buying a wireless access point or combined WAP and ADSL modem, it is well worth the extra to get one with a built-in hardware firewall.
By all means add a software firewall as well, and ensure that AV is installed & updated regularly.
Belt, braces, chewing gum, string AND baling wire!
BTW - if you are likely to want to share files between 2 PCs or print over a wireless network, you will notice a difference between g and b. Believe me!
SD