There's some good advice on this thread, especially Sheldon Brown's list of priorities, even though it refers to upgrading a hybrid. Pasted below:
http://www.bikeforums.net/showthread.php?t=270208&highlight=weight
Originally Posted by dydst Hello all,
I've been riding my 2003 Giant Cypress Dx (hybrid) to work and for pleasure for some time now, approximately 100 miles/wk. While certainly not a "great" bike, it meets my needs, and has really grown on me.
I was thinking about modifying/upgrading the bike. What would give me the best value for the money?
For instance, I was thinking about upgrading/replacing the derailers. Would that noticeably improve my ride?
(Sheldon):
Replacing the derailers would accomplish nothing, just throwing money away.
Here's what I would do, in order of priority:
1. New tires. Something light and smooth, maybe Avocet Slicks or something similar, probably narrower than the stock 40 mm tires, unless your local roads are very poorly maintained.
2. Clipless pedals and matching shoes. I like the Shimano SPD system a lot.
3. New saddle. Something narrower, firmer. Maybe ditch the suspension seatpost too.
4. Handlebars/stem. Something perhaps giving a slightly more "aggressive" position would be appropriate for someone who rides as much as you do.
The adjustable stem is heavy and clunky, and once you figure out where the bars want to be, you really don't need the adjustability any more. Might want to lose the riser bars.
Bar ends for additional hand positions would also be worthwhile.
Sheldon "Upgrades" Brown