Title says it all. Will this be noticeable if I only listen to these songs on my iPod or iMac?

Depending on the quality levels you select, the type of music you choose, the headphones you use, the acuity of your hearing, and your listening environment, the transcode might be noticeable to some people, but not to others.

Is this a useful answer to your question? Of course not. So I suggest you try converting a couple of tunes yourself at different bitrate-to-filesize ratios and then compare them on your player... if you notice a difference -- and if the difference matters -- then you've answered your own question.

in the past, it was possible to strip the DRM from the aac files. isn't that possible anymore? it means you wouldn't have to transcode.

It is still possible to losslessly strip the DRM from iTunes Store purchased content. We can't discuss it here though, we can only say that it is possible and leave it at that.

As BobO stated, try this for a couple of files and try to determine if you can distinguish the difference between the source AAC DRM file and the transcoded one. If you can't hear a difference then you will be fine. If you can hear a difference then you know that transcoding is not an option and you need to do some Google Searching.

DON'T DO IT! LIVE IS WORTH LIVING!

Seriously, I did this to a burned CD of Bach's Mass in B Minor, and everything sounded like it was underwater. Quite shockingly bad really.

Thanks for the reactions. I will try both options.