This is a UK site, so difficult for most of us to give good advice on dollar purchases.
As scaryjim said, 7750 is the highest card you could try. It is very new, right at the top of your price range.
It looks like the Nvidia GT430 is a 49W card, so a bit gentler on your machine. Not as fast, but also a lot cheaper. Although this isn't as fast, a quick web search on Flight Simulator X says it is a late 2006 game so by those standards it is a pretty fast card.
Personally I would go with the cheaper card, and save the money. It looks like if you want faster frame rates you would need a faster CPU for that game. For the upcoming Microsoft Flight, it looks like your CPU is below the minimum requirement
