Depends on the game and your gaming resolution. Some games, you won't be able to run on Ultra settings at all except for maybe a 800x600 resolution. Other games could get you Ultra settings at 1920x1080 for several years.
a year or two if my 6870 is anything to go by, and that's excluding the crazy ass demanding games. they are just going to get more and more demanding through the years.
OCing helps, but depending on the game, not by a whole lot. i suggest you get use to making compromises if you're not into buying mid-range GPU's every 1.5 - 2 years. most games at high at least looks better and run smoother than consoles.
The system itself is fine, however the video card is already aged a bit, if you want a system to last you 3+ years you need to get a dual video card system. No single gpu video card is going to last you very long if you intend on having all games at ultra resolution at 1920 or above.