Synching to a new computer

Hello all,

I have an iPhone that I have music on synched to a Windows PC. I just bought an iMac and want to synch my phone to the iMac instead. However, when I plug it in to synch, it wants to wipe my iPhone and use the music from the iMac (of which there is none). I want it to go the other way.

Now, my wife and I share the music library (she has an iPod, I an iPhone) from the PC. Is it possible to have the music library on both systems and allow us to synch to whichever?

Sorry if this isn't clear.

On a side note, I purchased a song on iTunes from my iMac and I cannot get it to download onto my phone from the iTunes store app on my phone. No clue what's happening there.

Thanks in advance.

mark