So I have a mac pro acquired somewhere in early 2007, I believe. Quad Core with a Radeon X1900. What are compatible cards for it today? I mostly use it as a Windows box. I believe I heard I can use special drivers to enable Crossfire? I could then a get newer ATI card and pair it with the X1900, right? Trying to beef up my games since I just added a very high-res monitor. Thanks!