luckyphil - nothing bighitter said was incorrect.
josh123 - if you are getting XTR rear derailleur, make sure you hunt down an M952 (ie: non-rapidrise) unless you are keen on the idea of rapidrise. Again, if you are buying XTR I assume you know the difference.
As for shifters, the SRAM rocket is an X9 equivalent that is compatible with shimano RD's (and attack is the X7 equivalent), if you prefer to stay with the dual thumbshifters.
However my recommendation would be a righthand XT, Saint, XTR, or even an LX. They all work great, and will keep working great forever.
Either way you'll have to buy a new shifter (obviously). And yes shifters make a big difference, because the "brains" as such are at the shifter end - a highend unit will make all the difference in your shifting. Money spent at the derailleur on the other hand usually goes towards lighter weight and higher strength. (XTR, X0, etc)