Grobi,
It can be done, but it isn't necessary if it is extra horsepower you are looking for

I am an FS addict and just purchased my first Mac Pro a week ago. With Bootcamp, I have XP running FSX with Acceleration package (sp2). I have a beautiful AMD9600 Phenom Quad Core PC with 4gigs of RAM and an 8800 GT (one month old and necessary for the business). It does about 7-10fps in downtown LA or Vegas or Seattle (higher rez cities) going low and slow.
The Mac Pro with the 8800 is a completely different animal. Although I have 10gigs of RAM in my machine...it is configured in a such a way that XP only sees 2! (I know I can fix it to see 4, just haven't changed to config yet). STILL, FSX is drawing 40-55fps in any of the high rez cities at 1920x1200x32, with all settings maxed...Still messing with Mesh complexities. It is insane...I have been flying Microsoft Flight Sims for 20years (first on an Apple IIE, followed by a 286), 19 on PCs...my first foray back to Apple and if your only game you play in Windows (like me) is Flight Sim...this is IT...the HOLY GRAIL!
BTW...I am not a frame rate junkie...but playable frame rates are hard to achieve on any Microsoft version of Flight Sim when first released!
And in reply to your first question, you CAN hook up two monitors with your one 8800 or grab the Matrox To-Go (I think that is what it's called) and hook up three monitors through the single graphics card...check out flightsim.com for details.
Good Luck and Happy flying!
Jeremy