- if nVidia it's SLI, if AMD it's Crossfire. and a single HD 6950 or HD 6970 will do the job on ultra, but you still can CF them for the better FPS.
- like said before depending on your CPU, PSU we can recommend a single or multi GPU, and what resolution will you be gaming at ?
and i would go to for the HD 6950 2 Gb version not 1 GB, the more Vram helps a lot in maximum resolution and will be a more future proof for the upcoming games.