I agree with the above posts, a 1200 bike will be much better than a 800 ride (makes sense). the $1200 probably wont need any upgrades right away where as the 800 may. Id also suggest alum frame, carbon fork. Id say the shifters are going to be the most important item NOT to skimp on, 105 would be great but tiagra will suffice, stay away from sora. Wheels also help maximize your purchasing power, get the best available package bundled with the bike, you'll save cash (do alittle research beforehand). deraileurs are easy to replace and not so expensive. all cranks will work fine just the expensive ones are lighter.
You will be amazed at how much smoother, faster, and nimble you will feel compared to a fat tire mtb. Most shops will have a few left overs from last year so you can bargain with them, your doing them a favor by clearing out old inventory.
Shops will also be able to swap some items for you at little to no cost such as saddles and stems. if they wont accommodate you, go elsewhere and let them know why your walking out the door, they will stop you. you are the boss!
And lastly make sure you leave yourself some cash because you will probably need a few extras like pedals/shoes, a helmet, padded shorts, water bottles/cages, seat bag, lock, rear blinky light....
good luck and dont let them push you around.

Pretty aggressive I know. Im really not a prick.