Also at least five years. From the previous example, the five year old HD4850 is about as powerful as a HD7750. That card can play modern games, all on "decent" settings. No, they won't be anywhere near "SuperUltraMaxOhWOW!" but they will still look good.
One of the major factors that has increased the power needed in a gaming GPU is the resolution. If your resolution doesn't change (e.g. this is a laptop), you may not need anything stronger. When the HD4850 was new, I believe I had a 1440x900 monitor. On that same resolution, I probably could still play many modern games on "high" settings with that card.
Where you might encounter [limited] difficulty is the DX version the card supports. Will DX11 still be sufficient in five years? Well, DX9 is still in use...
K thanks, what I really want is like to still be able to play games in fiture years, but i dont really wat tra graphics, just b able to play them.
So im going to get a NVIDEA 675M with Windows 8 because i want PhysX from NVIDEA for Arma 3, and i also learned that i can replace my card in the future so yeah, thx.