WMA lossless is a mathematically exact copy of the cd at about 1/2-3/4 the size.
This is the format I choose because mathematically lossless ripping means never having to rip more than 100 cds again.
When I want to load it onto my MP3 player, media player automatically downsamples it to whatever I want.
At BELOW 128kbps, wma produces far superior sound than mp3 for your "mp3 player" space saving awesomeness.
Media player 9 is great.
edit: microsoft also provides a free tool to convert back to the original pcm cd files!