I bought my 4890 a year and a half ago, and I'm not planning on a replacement until the 470 drops in price. I told you to replace them every two years, not every year. The 460 is a really nice card. Not mid-range for 2009 but mid-range for 2010 (when it came out...). Considering that games are barely starting to require the low end 2xx nVidia series chips is indication enough that the mid 4xx chips won't be out of developer's eyes for a while. Consider this, The developers can't logically develop for future technology, since future technology isn't available for them to develop on. I would imagine that the games just starting to get tested are being tested on the 4xx nVidia series and POSSIBLY the 6xxx series from ATI. You won't have to worry about a single card not being able to run a game for 3 or so years, since that is the approximate development cycle for a game (some exceptions of course do exist). Now if you want excessive framerates in current and most future games to show of and hide inadequacies in one's self, that is when SLI comes in handy.
Believe me, I do some Game Development (Rarely, but I know how things work), Video Editing, 3D Rendering, Photo Editing, Audio Editing, and gaming on my system. It still runs everything to my liking a year and a half down the road with a single card and it will continue to run things to my liking for another year or so. Many of my friends also Game Develop, and Render 3D ect. on older Qxxxx systems and some are unfortunate to be doing it on their work on C2D systems. I know for a fact you will never need any SLI, it is a waste of however expensive the second graphics card is, and it also normally costs you extra for the PSU and on top of that extra in utilities. You are kidding yourself if you think you can future proof at all, yet alone with SLI/CF. If you are going to pop the cash for SLI right off the bat you will be itching to upgrade that SLI to CF with the new 6xxx series, and then with the 6xx series for nVidia (assuming the 5xx is for laptops like the 3xx was). Be realistic and save your money.
One thing, you shouldn't poke fun at mfenn, who is genuinely trying to help. You just keep insulting his advice just because he doesn't give a paragraph explaining himself. The info is accurate, you just need to do a little research yourself into if it's true (which you should to begin with anyway). Second, you should actually review the past of graphics cards, and how buying a midrange card around $250 every year is not like buying 2 $250 cards and a more beefy power supply with the additional utility cost. Third, Like mfenn said, nobody has a crystal ball. We can't predict when your board is going to fail. I can tell you My dad has an almost 13 year old dell which is still kicking stock, and I have an HP sitting here that's 5 at Christmas time stock. Fourth, I don't think you have sound logic thinking that a midrange card half way through 2010 won't run a game when new years comes around. nVidia would be devistated, completely wiped out. You have no reason to be worried about single cards. Developers are only a gen. ahead at testing if they have an immense budget and great timing, so theoretically you should be able to run games on the 460 for two years. Of course nobody can guarantee it won't break, or it will get you 30FPS in Crysis 4 coming out in 2015. If you can't handle that then just don't get a computer. I'm tired of people coming in and telling someone who is taught to be a game developer how they need SLI or CFX. It's a gigantic load of crap fed to the consumer by graphics card companies to bring sales numbers up.