Worth upgrading?

Quoted post said:

Originally posted by iplanet
...one the bike is a bit heavy (15kg), ....Is it worth upgrading?

Unless you are able to get parts way below retail price there's not much point in trying to upgrade to get the bike lighter. You basically have to change every component to a lighter more expensive one to make a difference. And even after that your current frame can be as much as 1 kg heavier than a frame from a more expensive bike. It will probably be cheaper for you to buy a used lighter bike an do some maintenance on it than trying to reduce weight on your current one.

Upgrading for performance/functionality however is still an option.

Quoted post said:

I then realised that my rear cassette was 7 speed and these things come in only 8/9 (?). Which means a new cassette (hub?), chain, shifters and brake leavers no doubt.

You can get new derailleurs without having to change anything else. If your shifters and brakes are in the same pod and you want to go to 8/9 spd, then you'll need to replace both. If they are in separate pods you can keep the brakes.
You will need a new chain and cassette, but then there are some options available. Check http://www.sheldonbrown.com/k7.html for further information. You might get away with a body transplant (the part that holds the cassette), which would save you from having to replace the whole hub.

Quoted post said:

Originally posted by dabac

You can get new derailleurs without having to change anything else.

The problem was I couldn't see any decent (LX / XT) stuff that was 7 speed, it was all 8/9 speed.

So will a Shimano LX RD-M570-SGS Rear Derailleur (for example) work with a 7 speed?

I think you would get better shifting with better shifters instead of deraillers, they do work together but the shifter is the smartest piece of machinery in the equation.

I think I would try to pick up some altus or alivio (can't remember which is 7sp) shifters.

Quoted post said:

Originally posted by iplanet
The problem was I couldn't see any decent (LX / XT) stuff that was 7 speed, it was all 8/9 speed.

So will a Shimano LX RD-M570-SGS Rear Derailleur (for example) work with a 7 speed?

Unless you go for the new ones with the reversed return spring, the derailleurs (AFAIK) all have the same basic geometry and can be combined every which way. A 9-spd derailleur will have the same span of movement as your old 7-spd. The indexing is done in the shifter, as long as that one matches your cassette you should be OK.

I prefer to spend money on the the derailleurs instead of on the shifters. In muddy conditions I feel that this gives better functionality. In less taxing conditions you might appreciate the smoother action of a higher rated shifter more though.

dabac has the best suggestion. Sell this bike and buy a used one with 9-speed.

Quoted post said:

Originally posted by hedgehog
I think you would get better shifting with better shifters instead of deraillers, they do work together but the shifter is the smartest piece of machinery in the equation.

I think I would try to pick up some altus or alivio (can't remember which is 7sp) shifters.

Altus, fyi.

LX derrailleurs will work with 7 speed since the shifter is what determines the movement. The range the derrailleur moves is the same with 7 speed. If you want a more serious mountain bike I would recomend you just buy something better. You will spend as much, if not more money upgrading that one to better componentry.