First let me qualify this by saying I am not speaking as a STAFF member. I have not been active STAFF for quite a while due to some unfortunate personal circumstances. But I get sick and tired of so-called "experts" putting ATI down when they really have not done any testing of the product as an engineer would!
Second, People on this forum are depending on solid advice from professionals and users who truly give qualified not biased opinion. Don't diss a product unless you fully understand it. You can't believe everything you read on the Internet including reviews. Reviews can be biased and if you have ever tested anything you know that you can test with the outcome already know if you know the strenghts and weaknesses of a product. Unfortunately there are many amatuers who build a computer and think they really know what they are doing because they got lucky once.
Third, FSX is a finicky, patched up, monstrosity that just happens to look and run really good when configured properly...by the way, I like it! I recommend using Nick N's FSX guides for that info. I am aware he is biased toward Nvidia but I have gotten just a good results with ATI. I have posted many pictures with fps speeds on this forum to prove it.
I'm not sure just how ATI got such a bad reputation for FSX but it is totally unwarranted. They had one driver in the last year that would not run X-PLane(not a great loss in my opinion)
I test for TechCorp every other month and we test ATI and Nvidia cards in all kinds of configurations. James(An ex-NASA Systems Engineer with an MIT degreee) and I are the only ones who test cards in FSX and other games. All the other engineers test only other hardware equipment speeds with video cards. We have yet to come across an ATI card that does not render FSX as nicely as the Nvidia cards if not better in some cases. It is all in the setup. You have to know what you are doing with Catalyst and/or ATI Tray Tools to make them perform. I had listed a guide on here how to use it. You use it with Catalyst loaded. Besides most of your speed inside of the flightsim comes from your motherboard, CPU, memory configuation anyway.
In spite of the CRAP I read at Tom's hardware, I find the HD4850, 4870, and 4890 run FSX as well as the Nivdia 9800 series, GTX 275, 285, and 295 in FSX. I own an HD4850 on a Gigabyte EP45 UD3P motherboard, E8400 processor with Windows XP64 oc'd to 3.8Ghz, and I own a Gigabyte X48-DQ6 with E8600, and an Nvidia BFG GTX285 OC clocked at 3.8ghz. The 512mb ATI HIS HD4850 renders as much and as nicely as the GTX285. It also is capable of running the same frame rates as the GTX285
in FSX only. If yours is not doing that, then you are doing something wrong. It is not the fault of ATI.
The newest Catalyst drivers are better for the HD4850, HD4870, and HD4890 but some of the HD3850s run them without a problem. Nvidia, however creates some of the crapiest drivers I have ever encountered, except for the latest three sets. And in FSX you really need to use nHancer to fully enjoy great screens with FSX and most games. Either way you decide to go with your video card take the time to learn how to get the most out of it. If you are using and Intel board that does CROSSFIRE you may still use one Nvidia card if you like and not concern yourself with CROSSFIRE. On the X58 boards most them will allow you to go CROSSFIRE or SLI, a really nice feature! However in FSX it does nothing for you. Blame Microsoft not ATI or Nvidia for that.
It has been a while since I stepped on some toes so this has been a long time coming LOL. I will step off my soapbox now.
Ted
. 
