IMO, the 6990's are a terrible investment, unless you just don't have room to have several rigs. Going with a midrange card, (ideally a 5830/5850/5870, though those are difficult to find for decent prices anymore) will generally give a much better MHash/$$$ rate. As it is, you are lucky to get 1Mhash/$ with a 6990, but a card like a 6950, which can get 400MH/s with a bios flash, is nearly 2Mhash/$. Of course, system costs must be taken into account, but with the spendy PSU's required for a 6990, I just don't see the point in spending the coin on them instead of a cheaper alternative that gets more MH/s.
That's not quite true. The 6990s pay themselves back just as quickly, have a higher production/watts metric and can give you a higher rack density if you're living in a country where rent is comparatively more expensive than electricity
Also, the 6950 that can be converted into 6970s got sold out something like 10 billion years ago when everybody and their grandma heard about how they could save 150$ on their video cards. The newer 6950s can't be made into 6970s anymore. If you were to buy new cards, you basically can't find 5870/5970s anymore so you have to move back to either 6870s in some kind of optimized setup or you spend more and get 69-5/7/9-0's.