16x/4x with crossfire or sli is bad
just stick with the single gpu, so you are always compatible with new/old games and then upgrade to a more powerful single gpu.
I was running dual 6950s (at 16x/8x), one failed (died) on me so I ended up replacing them with a gtx 670 (performance wise I switched from dual gpus to a single gpu and kept the same performance) E.G. a single more powerful gpu. Currently running two 670s ;o