I'm not currently planning on adding another 5970, but I've tried to leave room so that I can if I later decide I want to do so. I mostly play 2D games (Starcraft, Warcraft III, Civilization IV) and only a few 3D games (Everquest, occasionally Team Fortress II). So I probably could get by with just a 5870, but I wanted to be sure.
That, and I plan to check out one or two of the newer games -- Batman: Arkham Asylum and Bioshock come to mind. But still the 5970 should be able to keep up. If not, I'll scrape up some more money and buy another card, another water block, another pump, a different reservoir, another radiator, a couple more fans, some more fittings, and then try to figure out how to customize the bottom of the Corsair 800D to take a dual radiator.
The 980X actually added over $800 to the build cost. I would otherwise have bought a 930 for $199 at microcenter, instead of a 980X for $999 (which was $1072 after taxes, bleah).
Total cost for the whole thing, so far, just over $7000, including the three 30" monitors. I haven't totaled it all up recently, but then I don't want my wife to focus on the cost, either...