System Advice

personally i think after you get this rig you will want to play cod and stuff on your pc instead of xbox.
Looking at your build you might want to bump up to a 80gb ssd since 40gb will only really give you room for windows and a few applications/1-2 games.
To save money you can use the 1tb hitachi/seagate hdd's since the 64mb cache won't really make a difference.
you can do the same with your 2tb drive
and that comes close to covering the 80gb ssd (i think)
with the blu ray player heres something i bet you didn't know. blu ray and pc's are complex so to be able to play blu ray on your computer you have to make sure your monitor is HDCP compliant. That is something that is not yet a standard on monitors so be sure to check.
the differance between the gtx 460 and 480 is that the 480 runs great in 1920x1080 which i'm guessing is the resolution of your monitor since its 24 inches. I would say its worth the upgrade because it will make your gaming computer last longer and will play better in 1920x1080.
Unless you are going for the asus sound card because of experiance or something you should be fine with the x-fi titanium fatal1ty and you can save $45.
With noise reduction you selected you don't really need it. the fans on the haf 922 are very quiet and the only time you will really hear the fans are the ones on the gpu when you are gaming.
you can go down to windows home premium and you won't miss any features you probably would use.

with that you have a $2509 build. its only $9 over and i would consider that barley in the budget.
Heres the new build with the changes i said.

now for you questions
The computer on all day won't be a problem just it might waste a bit of energy if your not using it. also just to note it takes about 15 seconds to turn on with a ssd so its easy to turn on and off.
with the cpu fan you selected you will be fine.
As i said above the gtx 480 is much better because it runs games in 1920x1080 great and thats the resolution your monitor probably uses.
Yes 750w is fine for one of any of the nvidia cards for sli you will need a bigger psu though.