I dunno, it's kinda like recording a tape from an FM station, then dubbing another copy for a friend... just starts to get shitty :-/ It may not be immediately noticable, but generally you'll start hearing "tinny" or "watery" sounds from the higher pitched notes, or a complete lack of any of the lower notes. It's especially noticable if you play on even a halfway decent stereo, be it in the car or at home.
I play 128kbps mp3's I ripped from original CD's, both on my stereo, and in the car, and the "watery" sound is pretty obvious on some of the stuff. And neither of my audio sources are anywhere near "high-end", they're just average, mid-range, puts out a decent sound and don't really need anything more.
It bothered me enough that I went back to the more definitive CD's in my collection that I will always have (ie: I used to have 500+ cd's, but after ripping them all, I took 350-400 and sold them to used cd stores, keeping only the ones that are my favorites), and re-ripped them at 256kbps. I acutally test ripped a few at 320kbps and 256kbps and with my setups, I couldn't hear any differences between the two, so didn't see a need to waste the space on 320kbps rips. If someday I get better equipment, and again am not satisfied with the 256kbps rips, I'll re-rip the CD's again at 320kbps, or possibly even go with something lossless like flac when I've got the disposable HDD space available.
128kbps seems to be pretty good for generally anything, but to be on the safe side, anything new I get, I go for 192kbps... unless it's a new album from one of the bands whose music I deem worthy of 256kbps
I'd get CDex from the link whitesmoke posted, and use that to go from the CD to mp3, rather than CD to wma to mp3. The fewer recompressions, the better.