590 will be out (announced... possibly not available for retail) within a week, last I heard. They had to correct some last-minute issues on some capacitors on the latest batch, or something of the sort.
Not sure on the price-side of things, but I'd expect the prices of the 5xx series to drop over the next month or two, based on historical market prices. They're not the "hot new cards" now, but now the high-end mainstream, so they'll lower the prices to attract more buyers. It could also very well depend on ATI's product releases. ATI usually undercuts nVidia, who then lowers their prices on comparable cards.
If you're looking to buy sometime in the next two months, wait a bit on the 590. There's often driver issues and growing pains on new cards making them a nightmare to work with, plus the price is extermely high for the first month or 6 weeks.
If you're looking to buy anywhere from now to the next month, get the 580. I think you'll find you won't need to SLI
it.... but if you decide to, you'll do better if you get the exact same model, so watch the model numbers closely and if EVGA stops making one, snap your second one up fast.
The above post is my opinion. Don't like it? Don't read it.