With WiFi make sure you turn the encryption and make a strong password. I would still put my LAN connected machines behind a firewall.
In addition to the above great advice I would also recommend:
1. If it's not too late, keep your DSL Router and your Access Point (AP) in separate boxes, connected by ethernet.
2. Please change the default name and password of both your Router and your WIFI (AP). Most of the WIFI's around my house are named "Linksys" and, since I know the default Linksys password, I can log into their Routers and make any changes I wish (if I wanted to).
3. Pick a non-standard IP numbering for your LAN. Something other than 192.168.xxx.xxx.
4. Set your AP to connect to only MAC addresses that correspond to LAN devices (computers, etc.) that are yours.
5. Set up static IP addresses in all your LAN devices, and set your Router to NOT hand our dynamic IP addresses upon request.
These steps will help
keep hackers out of your network. I never worried about this stuff until I got WIFI for my wife's laptop, but these are just good security practices.