The old 2.66GHZ Yorkfield processor is already 'obsolete' being a socket 775 CPU. Intel has moved on with several sockets and chip sets since then. Having said that, the old Core 2 Quad is still a good performing processor that can game well with a good graphic card and the fastest RAM your motherboard will accept. And for everyday computer use, it won't slow you down. But 'future proof'? It's too late for that.
It will last as long as you believe it is providing you with enough performance. My Q9450 @ 3.0GHz still provides me with enough performance. I plan on upgrading to Haswell or to Broadwell (2014). If I decide on Broadwell, then I will increase the OC.
For general daily use, web browsing, listening to music, watching video, a stock speed C2D E6300 would be more than enough. "Moderate gaming" varies from person to person, but if you are not looking to max out everything and add a powerful graphics card that could potentially be bottlenecked by your CPU, then you should be fine for the next few years.
Agree 100%. The Yorkfields were a good processor. I had my Q9550 for a long time, and just recently sold it for $217.50 top bid on ebay. The last card I had used with it was one of my GTX 570s when 570s were new. Even struggling against 800MHZ DDR2 memory, it allowed the card to game with high frame rates. And I never O/C it or the board.