For upgrades, what do you plan on using the bike for? If you upgrade to the SLX or XT systems, will you be using them as a 27 speed or 30 speed? Will you be doing more road or mountain biking? If you upgrade to a whole system, know that the gear ratios will change. The stock gearing up front is 26/36/48. For the SLX and XT(MTB based) it is 22/32/44. Better for climbing but much slower for road.
Another thing i have learned from my DS is that the Fork definitely does not have enough travel for heavy mountain bike use. probably the max i would do to keep it safe is Single track. I have taken mine off 35+mph downhills with 4' jumps at the end of them and although i make it..i bottom out so much i am starting to hurt my wrists. It needs 80-100mm worth of travel. But keep in mind, once you start getting longer travel forks, the geometry changes and it will be more twitchy on steering. The stock tires also need to be upgraded to wider tires for more grip when you are going on high speed downhills.
As for Road, there really isn't much to upgrade but tires. It is just as fast as a roadbike, but doesn't have the Aero's of a road bike..
At this point, you might want to ask yourself, "Do you want to upgrade to make it handle like a mountain bike or do you just BUY a mountain bike?"
I upgraded my DS 8.4 to match the components of the 8.5. I have the Deore shifters, Crank, SLX rear derailer, and Shimano Brakes. SO far, they have been STELLAR! I didn't upgrade to a bigger fork because i decided im going to put the $$ into a real mountain bike and keep my DS a hybrid. You may or may not go this route, but it is one way to look at your upgrade goals. Im saving up for a Trek Mamba. =)
Best of luck on our new ride! What ever you decide to do, make sure it is right for YOU and no one else.