You should considering getting more than 2GB RAM for smooth gameplay.
A 512MB Card won't be able to get much on texture size considering your going to be limited on Video Memory, try going for a 1GB card from ATI. They should sell some pretty good 1GB or above card in the 47/48xx series.
CPU. Your choice, either get a Quad Core from Intel Core series or a 3 / Quad from AMD. Your going to struggle on performance with a 2 Core CPU.
If Money isn't a issue. A i7 would be killer if you had the money for the DDR3 memory and the X58 Board.