I don't own a wireless router (I use ethernet), but I know a little about what you are asking about.
Speed:
802.11b (Wi-fi) - 11mbps
802.11a - 54mbps
802.11g - 54mbps
802.11b & g transmit at about 2.4Ghz. Most cordless phones can change "channels" and shouldn't conflict. 802.11a broadcasts at 5Ghz, very few phones use this frequency.
You should look for a router that supports WAP encryption. This will help keep unwanted computers off of your network. You can also allow only specific MAC addresses onto the network, so it is even more difficult to get in. The method for applying these measures varies by manufacturer.
However, security is not a major concren, unless you are storing sensitive information on your computers. Most people who sneak onto Wi-Fi networks just want to get some free, anonymous 'net access.

Wifi routers are smart and can change frequencies to avoid interference. The signal can reach up to 100m

, depending on it's environment.
I hope this is somewhat helpful.
