I think I know the answer to this question already as mp3 is a lossy format but I am really looking for confirmation.

So as the title states, just say I burned some Mp3 files to a music cd using Nero for example.

If I rip that music cd which was originally burned with mp3 files to AAC, would there be a quality loss on the original mp3 files and how significant is that loss in realistic terms.

Any input would greatly be appreciated thanks.

Yes, there will be a quality loss. Whether it is significant to you only your own ears can say. If you want to transcode MP3 to AAC there is no need to burn to CD first; just use an application like foobar2000 or dbpoweramp.

What is your goal in doing this? If for playing on an iPod, iPod can also play MP3. Or, if you still have the original CDs you'd be better off ripping them directly to AAC. Actually, you'd be best off ripping them to lossless like FLAC so you can convert to any format later with the best possible results.

I no longer have the original mp3 files!

Got it. You can maintain the existing "quality" of whatever is on the CD by ripping it into lossless. The trade-off is file size and some compatibility with players. iTunes and iPod do however support Apple Lossless. Otherwise, you can minimize the perceptual differences by ripping to a higher bitrate AAC such as 192 kbps or 256 kbps. Try it and see what works for you.