I would *love* to upgrade my equipment, but I feel like I get pretty good soccer photos with a Canon 60D + a 70-200 f/4 L (non is) lens. In my own case, if I had the budget, I'd upgrade first to a 70-200 f/2.8 lens, then to a 7d, then I'd add a 1.4x teleconverter and/or the sigma 120-300mm f/2.8. A 200mm lens on a crop body gets enough of the action on the field, but more reach would be AWESOME to have so that I didn't spend so much time waiting for the action to come to me. In your case, moving to a lens like mine would probably appreciably improve the colors and sharpness of your photos -- L quality lenses are really awesome, and the 70-200 f/4 can be found for under $500 used (I think it is around $675 new). You'll find yourself using it for portraits and just about everything.
You'll improve a lot if you follow the advice here and get out of sports mode. I shoot wide open (f/4... would love to be able to go to 2.8), Av or M, and watch the ISO to keep the shutter speed around 1000... might drop as low as 500 in low light conditions to avoid bumping my iso up too high. And like others mentioned, I shoot from down low -- it really does improve the look of the photos. I wouldn't worry so much about frames per second -- that just gives you more photos to comb through and toss. It's nice, but not as important as good glass.