The best bang for the buck is to go for lighter , better wheels and tires. If it is a V-Brake bike this is a good time ,because many folks are going to disc wheels. Many sets of rim brake wheels are being sold cheaply on ebay. Wheels are also transferable to a new bike-many parts aren't transferable.
If you derailleurs work ok, new derailleurs won't give you much bang for the buck.
XTR 517 wheels can be bought for $100 now(used), and they are very nice-smooth and probably 150 or more grams lighter(a piece) than your current wheels.
Luck,
Charlie