It's *supposed* to be confusing lol! If it weren't, everyone would buy the same card and the other guy would go out of business!
I think its generally recognized that the 285 is the fastest single-gpu card around. And that the 295 is the fastest single-PCIe-slot card around, tho it has 2 gpu chips meaning its already SLI'd.
They are both expensive, and in many games you can get better frame rates by using two lesser cards in Crossfire (or SLI). OK, you already know that.
Here's a review of a specific 285 (factory OC'd) vs 2x4870 across a variety of games:
http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/13847-evga-geforce-gtx-285-1gb-ssc-edition-review.html
Here's a line from the conclusion: "Performance-wise, there is no faulting this card (285) considering it beggars the GTX 280 while at times flicking the ears of the HD 4870 X2"
So 2x4870 is mostly faster as benchmarked, and costs maybe the same, depending. 2x4890 would be faster yet, and somewhat more expensive. And maybe overkill . . . today . . . depending on what screen resolution you are running
Here's a good read on the best graphics solutions at various price-points:
http://www.tomshardware.com/reviews/radeon-geforce-price,2323.html
Its out of date of course, prices have come down, but its a good starting point.
Short answer - If the difference in cost isn't a deal-breaker, and you want the best single card, its the 285. To which you can add another if growth is needed. If you want a cost-effective solution for today's games (not necessarily for MMOs) at your resolution, the answer may be 2xCheaper cards.