What are the actual system requirements for this game? Battle.net just lists recommended parts like NVIDIA® GeForce® 260 or ATI Radeon™ HD 4870 or better. I tried playing the open beta with everything except resolution turned all the way down and the game plays like I'm running in mud.

My system:
Windows 7
AMD Phenom II 2X 511 Processor ~3.4GHz
5120MB RAM
ATI Radeon HD 4200

I am considering upgrading my graphics card, but I before I go spending money on something I don't really need, I want to make sure this game is running slow because of my system and not something stupid like lag caused by blizzard.