I don't think a new card would really satisfy your woes. FSX is a very processor intensive game. You could slap the very greatest graphics card into your rig and you wouldn't get much of a boost. Long story short, you need a new computer with an Intel i7 if you want to really tear up the virtual skies. However, for your situation if you want a slight boost, get rid of all that traffic!  You only need probably 30%. Also make sure vehicle traffic is off, and turn down autogen density.