Well, although the general public accept that 128kpbs is close to CD quality, through a decent hifi, the difference is noticeable. At 128kbps Cymbals would sound "splashy" etc. Piano would not sound as "clean" as it would on CD etc...
iTunes+ files at 256Kbps are a lot closer to CD, and it takes a more expensive hifi to really notice the difference (say upwards of £750), although if you have really sensitive ears, the differences can still be noticed on an ipod with a £50 pair of earphones...
Basically - even though you are putting the files back into CD format, you won't be able to replace what has been lost when the file was compressed down to 128Kbps.
Most of the people on this forum tend to rip their CDs to Lossless (either Flac, Windows Media Lossless or Apple Lossless) to get the best sound quality at a reasonable file size. But downloads are limited to whatever your download service has ripped them in - 128 or 256 for iTunes /iTunes+.
...but, if the 128Kbps sound fine to you, then don't worry about my ramblings - your ears should be all that matters to you!
Hope this helps!
FBSF.