sniffing and detecting with one WLAN card possible ?

Hello,

I am new to WLAN an I intend do buy a WLAN card and use it
under Knoppix/Debian.

As I understand in the past there were 2 types of cards:
one type can detect access points (even with external antenna
like the Orinoco)and the second type can snif (e.g. the ones
with PRISM2-chipset).

My question:
Is there no type/brand which can do both jobs? Or is it worth
bo buy 2 separate cards?


Any tip is appreciated very much. Thank you!

John