Has anyone seen a difference between the 2 for flight sim?
YES! There is a MASSIVE difference. You'll notice it right away. I had a 128 fx5200, but now i have a 256 X800XT PE. And the difference in Anti Alaising/Ansiotropic Filtering is Amazing! You will be able to run it at a better resolution.
BUT, if PC in question is not good on its own, then the card will be held back by the slow PC.
I was looking at an article in toms hardware guide abuot the Radeon 9800 pro 128mb and the 256mb card the difference in fps was tiny but you can run at a better resolution .You will proberly be better of with the 256mb version if fs10 is eny bigger than fs9