You may be able to go to the upper right hand corner and click on your user name(I have a paid monthly subscription so it may vary if your not a subscriber) and under the "general" tab uncheck "use lower quality files for streaming." Now I dunno if your connection speed will have an effect on that or not, but it should be noticable if it does. Also what I did is to right click on the player screen somewhere and bring up the Adobe Flash Player settings, and adjust the "Local Storage" tab to unlimited, or at least something high, so that Grooveshark can store as much as possible to my computer and playback from there. Initially the local storage setting was pretty low. It doesn't take a lot for a 192k file, but moar is bettar, no?
I like Grooveshark. Got a lot of new music there listening to Radio. Much bettar than Last.fm.