dual core or single core for interim PC + video card?

My computer has been sick for a while had finely passed away the other day. Its likely salvageable but not likely worth the cost. I was kind of hoping I could soldier on with my current system until Vista and DX10 comes out and then go hog wild on a new system then. However with the passing of my current system I am now thinking about building a budget interim system that I will use for about a year. Since my old system is 3 years old it should not be difficult. One thing that I am very confused about is the new dual core processors? These seem all the rage and very popular. On paper they seem to be the way to go. However it seems I have been reading how FSX does not really use dual core technology and a lower end dual core processor might actually run slower with FSX then a faster single core processor.Since I am not planning on loading Vista on this new interim computer and it appears FSX does not use much dual core technology would a fast single core be better on this interim computer?Also in my interim computer I am planning on going with a mid level 512MB card. Either GeForce or Radeon. I have always been a Geforce fan but on my last system I was talked into going with a Radeon card. I would say it was the single component in my system that made FS9 really shine at the time. I think back then the Radeon was a better card for FS. Is there a clear difference today between the Radeon and Geforce cards? Sorry for the basic questions but I have been out of the loop for a while when it comes to hardware.My budget for my interim system is $1,000 for the tower. So if I play my cards write I can have a pretty high average system put together.