Variable bitrate at 256 is acceptable. You'll find them just as good. Naturally you will lose some. Constant bitrate at a lower compression will clip the entrainment in places. at 320 you're more or less preserving at a rate that some MP3 players will not accept or play (CD MP3 players almost certainly won't play it. but the bit rate is that high it would be pointless recreating CDs anyway.)

Alex