I second a fast prime, you can spend around $2000 on a 5D II or the recently anounced 6D and only get 1 1/3 stops of light or so. Or spend $500 or less and get 3 to 4 stops depending on the what focal length you comapare it to! Pretty good value eh? Obviously there are other reasons to get these bodies but lets get into that right now. We are talking budget not the cost no object.
As far as the body is concerned the next body worth upgrading is the 7D, the 60D is to close to what you have. The 7D won't help with IQ but it will help alot with FPS (how many pictures a second it takes) and AF (auto focus). I will allow you get more keepers by getting the perfect shot (because of FPS) and in focus (AF) but not IQ (noise levels).
John.