I don't know about compatibility, but if you're upgrading you should definitely go Sandy Bridge over a 990. Why would you spend a thousand dollars on a processor? For that price, you could get a 2500K and a motherboard to match, with a beastly graphics card (or two) into the bargain!
Sandy Bridge CPUs are roughly as good as the 990 when you overclock, and cost $750 less.
Wait for a month or so for the Bulldozer, if the leaked specs are correct, an 8-core BD should be equal to the 990x and should be much cheaper. You will need a $100 motherboard and probably new RAM if you don't use DDR3 now.
1-if by 3d u mean ivybridge then you will have to wait quite a while they will be out early next year
2-dont buy 990 as it is extremely expensive and the 2600k performs almost as well at 1/3rd the price. 990 has a really performance to price ratio in comparison
3-if you really want to buy intel's enthusiast class processors then wait a few months sandy bridge e will be out then at least you will get value for money