Whats the best video card i can get?

I have a Dell Inspiron 530 (not the slim) it has Windows Vista Home Premium, 6g in RAM, Core 2 Duo Processor at 2.80GHz (2 CPUs), it has a 750g hard drive but only like 300 somthing left, it runs at a 64-bit (not sure what this means). For my current video card that came with it is an Intel GMA 3100 (which sucks), and the approximate total memory is 286 MB. It also has somthing called DirectX. All i need to know is what is the best possible video card i can use. I mainly just want to play Halo 2 Vista but being able to play other games as well would be amazing.