I have a desktop PC that is around a year old and I'm a pretty avid gamer. I'm looking to upgrade the GPU, as I am currently running a ATI Radeon HD 4670. The desktop has an Intel DP55WG motherboard with a Core i7 860, 8GB of Corsair DDR3/1333 RAM, and a 500 watt power supply. With that in mind, if I'm looking for an improved 3D gaming experience what is my best option that will preferably cost me less than $400 for the upgrade? Thanks.