I've built the better Ramsey AM transmitter and was disapointed in it's sound quality and noise level. I have a feeling these are probably better:
http://www.ontheair3.com/ As far as FM goes, Ramsey does offer a decent FM Stereo transmitter for around $250. For those on a budget the C Crane/Sangean digitally controlled FM Stereo transmitter is hard to beat @ $69.95
http://www.ccrane.com/fm-transmitter.asp I own one of these and it works great!