Wifi mac security

Hi all,
and thanks in advance for your help.

I would like to ask some questions about Wifi Network 802.11b/g and 802.11i

Let s think we have a company that is giving access to its employees to an access point through an authentication key.
To connect internet then all the stations must pass through the access point.

I want to know:

- can i forward the packets from the access point to a monitor analyzed? are they decrypted?

- what can really do one station of the Wifi network trying to spy/sniff/steal info to another station?
I ask this because this malicious station have all the credential to be part of the network, so should it pass just the firewall of the second station to get into the other machine?


I would like to clarify this aspects.
Thanks
Nick