There's no better site for music information than the All Music Guide. It contains a huge database of artists and just about everything you might want to know about them, including biographies, discographies, in depth reviews, music samples, chart information, similar artists, influences, and a whole bunch of unique features like "Explore by Mood." You have to register (for free) to get at all the good stuff.
The site is very usable, with a simple tabbed layout for each artist. I really like being able to see ratings for all an artist's albums at a glance along with similar artists an influences. Coupled with a service like Rhapsody or iTunes, you can ensure the albums you buy won't disappoint.