hot damn, that's a nice bike. I wouldn't worry about upgrading cassettes, you can do that when they wear out. (well I guess you can upgrade anything when it wears out, but whatever).
Yeah, so you bought the wrong bike if you want to upgrade. That's a sweet bike as is. It's xt and lx, and the only thing better in shimano is xtr. If I were to upgrade one thing without trying it, I would probably do the brakes because I have only heard bad things about htem.