Worth the upgrade? Using FS9, high AA/AF setting, contemplating if to install FSX, since my 8800GT doesn't really chew through FSX as expected... Any noticable performance boost? Specs see below... (price is now 150€ on ebay, expected to go higher...)

I have used an 8800gt and a gtx 260 (just a tad slower than your 9800gx2) with my quad. There is a difference, but it is not huge. Things are smoother. Your jaw may not drop, but I can't say you'll regret it. For me, the jump from a dual with the 8800gt to the Q9550 and the 260 gtx will have to be my last for a while. Too much money in time like these. Both were money well spent.Bob