Companies only put parts together - what you want to know is what specifications do you want to run it smoothly.
You'll definitely want a decent quad core. Flight simulators are really processor and RAM intensive, as dadiggle said, so I'd probably recommend 8GB of RAM to go along with it. Ontop of that, get a graphics card that will suit your screen resolution - I'll leave that recommendation up to the rest of the forum.
i have flown most of the microsoft flight sims with add -ons...i am running 2.6 intel quad.. 8 gig ram..with a mid range nvidia vid card with 2 gig ram can run fsx at about 75 percent full effects but you will still see objects populating when you fly over 200 mi per hr. at 4 or 5 thousand feet thinking of getting 3.9 processor and 16 gig ram from costco make sure your compuuter has hdmi so you can hook up your big screen to your computer...there are not to many computers out there that can handle fsx to its full capacity... but i seem to have more problem with my train simulator that really has bad frame value.. and now my computer is starting to over heat gotta put a ext fan on the open side of computer lol