6950 toxic has a bios switch (no flashing required) that turns i into a 6970 nothing required but the flip of a switch on the card. Toxic no contest, I might even just look at getting one 6950 toxic and using the 6970 bios (which again is already on the card)
CF and SLI will not likely be needed, If you are favoring Nvidia like rolli said the 570 seems pretty awesome and more affordable than CF or SLI.
Like I said just before, I do have a budget that I intend to use all of, so I want the best possible performance in gaming. I figure the toxic cards in sli might be the best bang for buck, giving me cfx 6970's for the price of 2 6950's providing it works