Audio line-in Q - old iMac and new iMac

Hi all,

I have a Technica headset with a swinging mic and a twin output, pink
and green (you can tell how much I know about this stuff ...) Also a
volume and on/off switch for the mic.

I cannot get the mic to work when I plug it into the line in plug at the
back of my old iMac from Xmas 2008:

Model Name: iMac
Model Identifier: iMac7,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.4 GHz

Then I found this discussion:

I hope to get a new iMac shortly when the new model comes out but in the
mean time I wondered if there is anyone in the know who can confirm
where I stand.

Is it correct that I am using a bog standard headset and my old iMac
needs a preamp ?

Do current iMacs and probably the new one still have that requirement ?

Is a USB headset/mic an alternative ?

My needs are basically skype and other online voice comms.

Many thanks !