What does your bike not do that you'd like for it to do?

Most component upgrades don't make economic sense. Shimano, for example, offers several mountain bike component groups. They all do the same things and they all function. As you move up the cost food chain they operate a little more crisply, they are a little lighter in weight and they tend to look a little nicer. Prices, however, jump up exponentially.

If your goal is just to jack with your bike, however, I fully understand because I do that kind of thing myself. My recommendation is to ask everybody you meet to find potential donor bikes. When you find parts that you think you'd like, bolt them up and see if you can get them to work. Just don't expect your bike to suddenly get noticeable faster just because you installed an XTR derailleur.