Killa, your choices all seem ok but if you want good FSX performance and can swing it, get a decent Asus Mobo as they allow for over clocking quite well. Right now you should go with a Sandy Bridge CPU such as an i72500K or better still a 2600K, also invest in a decent aftermarket CPU cooler. Look around and you'll be able to find either an ATI 5870 or nVidia GTX470 for a reasonable price and also up the PSU to an 850W, this will allow you to add a second vid card down the road for XFire or SLI & multiple monitors ;) Definately go with 8 gigs ram but if you do build a SB i7, all the ram in these systems runs at low voltage so it doesnt get too hot, so u don't really need the expensive ram with heat spreaders etc...
This should all fit in your budget and will future proof you for awhile. As for the total sub-total thing, it's probably taxes. Good luck on your build.
Oh also go with the 64 bit Windows7.
Hope this helps.