I am thinking of buying an iMac. Problem is, I have 12,000 songs
encoded on my harddrive in Windows Media Player's WMA format. I know
that iTunes has an 'import' function that converts from WMA to AAC.
What I want to know is, what is the likely reduction in quality (in
terms of bit rates). ie. Would 160 kps WMA file, once imported into
AAC format, go below the quality of a 128kps mp3 file (for instance)?

If there is any loss in quality, it wont be obvious.
AAC file compression is much better then MP3.  

Best way to expalin this is that a 128kb AAC file has the same audio
quality of a 160kb MP3.  (Both WMA and MP3 codecs are lossy.)

Also, its typical that a WMA file will have a better sound then a MP3
enoded at the same bit rate.

Itunes for the PC is a free download from Apple.  I suggest snagging a
copy, and import a few songs to see if you are happy with it.

Hope this helps you a bit.

-Phil