If I go for a short cage rear mech how many links will I need to remove from my chain?
I run a 9 speed system with deore front, XT rear mechs and an SRAM PC951 powerlink chain.
My XT mech looks as if it's on it's last legs.
Chain length is mainly determined by the sizes of the largest front gear and the largest rear gear, I use the Sheldon Brown formula = chain round both large gears (but not through the rear mech) + one link extra. The rear mech size is chosen depending on the size overall of gear ratio (it has to take the slack up when smaller gear combinations are used). I would think that if you haven't changed the gears you should keep the same size rear mech.
a short cage XT mech is actually a medium cage so really it ain't that much difference tbh
fit the new mech and as you are using a sram chain, you can join it up. put it in the smallest ring at the front and the middle of the block at the back. you can see if you need to remove a link or not.
You shouldn't really use the smallest front ring and the smallest cog at the back together so its pretty irrelevant what the chain looks like in these gears.
Full instructions here (including why the chain length is important) http://sheldonbrown.com/derailer-adjustment.html#chain
Note:- (copied from Sheldon Brown) "If the chain is too short, it will be at risk for jamming and possibly ruining the rear derailer if you accidentally shift into the large-large combination. Never run with a chain that is too short, except in an emergency."