When you look at individual components you're saving grams (454 g to the lb.), and every 100 grams can be very expensive. It can cost $1 per gram depending on the component!

$200 spent on derailleurs and a seatpost might save 1/2 lb, and you probably wouldn't feel it.

On the other hand, $200 spend on a decent wheelset and lightweight, kevlar bead tires will save 1 - 2 lbs. and most of that is rotational weight. That 1 -2 lbs. will feel like you dropped 5 when accelerating.

Suggested wheelset: Mavic 517 rims, 14/15/14 db spokes, LX or XT hubs. The difference between LX and XT is minimal. If you're on a budget, LX is fine. Save the money on the hubs and get a good set of kevlar beaded tires, Kenda Kharisma Lite, Hutchinson Python Air Light.