I've been looking for the answer aswell.. Don't go with either of those cards. From what I've read FSX prefers Nvidia, specifically the pre-200 series cards. I just bought a 9800GTX+ (haven't got it yet but it's apparently one of the best cards for FSX) although I got a bit screwed cause I found out later that a 250GTS is just a rebadged 9800GTX+ that runs cooler. So I actually paid more for an older/hotter card. Also I assume you have SP1 installed for FSX, otherwise it won't take full advantage of your quad core.