With Apple emphasizing Cover Flow more and more in the new iPods, etc., the need for good quality album art is ever increasing. I know iTunes is supposed to grab art for songs that are in your library and the iTunes Store, but in my experience it hasn't worked that well.
Fortunately, Josh Powell has cooked up a nice little site called Josh's iTunes Album Art Grabber that lets you search the iTunes album art repository and download high quality jpgs from Apple's servers. In my brief test (with Springsteen albums), most of the cover art came back as 600 x 600px. However, some albums featured art up to 1425 x 1425 or higher.
The search engine is rather strict ("White Stripes" returned nothing, it had to be "The White Stripes"), but this looks like an excellent resource for filling in holes in your collection.