Once there are other options in the System Preferences > Sound > Input tab and Output tab then iChat will show those sources.
Pic of my System Preferences and my Messages settings
The Mac has an Internal Mic but I am using a USB Mic.
On another computer running Leopard I have a USB 2.0 external Camera with it's own Mic and that is using the Camera's mic.
You can get USB headsets.
You can get 3.5 mm jack ones with a jack for Input and Output. This sort may be unsuitable as 3.5mm jack mic may also not work.
Mic are styled Lo-in Line and Hi-In line and a Mac needs them to be Hi-in Line (PC mics tend to be Lo-in line devices)
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously