Most people estimate the minimum amount of RAM neccessary for Vista is 512MB. Of course, it probably needs at least 1 GB to run smoothly like you said but I don't think I need MORE RAM. The OS is stored on the hard drive not the RAM card. We'll just have to watch it play out.
I was more concerned about my graphics card. I thought maybe I needed a 256 MB card. I think I'll be okay though, its a RADEON card so I have faith in it.
Also, when is DirectX 10 coming out? I heard there is supposed to be a 9.0d next...but I also heard that FSX only runs on DirectX10