I think he's talking about how long it will last without upgrading.
The only reason I bought the HD6870 was because I had reward cards to buy it with.

Its predecessor, an 8800 GTS 512, actually did play everything I threw at it and was even decent at BF:BC2. I could've lived with it even today but the upgrade itch overcame me. I think the next wave of games in the fall may be too much for my E8400 though, and between reward points and bonus gift cards at work, I intend to upgrade soon.
Although, you do make a good point about the rewards card. What it really boils down to is having a plan in place that will allow you to upgrade/buy stuff that you want responsibly. I personally have a set amount I set aside from each paycheck that is mine to spend on on whatever I want. It works out well because I know what my available funds will be so I can plan for purchases.
That's a good plan. I've found that as I've gotten older (and married

), my computer needs have diminished. When I was younger and single, I had 4 gaming machines in my house that were kept fairly up to date; now, I wouldn't even dream of that. Part of it is because I have consoles for some of the games we used to play and the other part is that I have new and different priorities and hobbies, like retro-computing. I find rebuilding and playing with the legacy machine in my sig more fun than playing with modern hardware in many ways.
Your post also makes another good point that shows how difficult it is to truly answer the OP's question. You appear to be happy gaming with a Radeon 6870, and the OP is worried about the longevity of his GTX 580. I'd imagine that your standards for what the two of you consider "decent gaming" differs.
For me, if I can play at over 30 fps with most eye candy on, I'm fine with it. Right now, the E8400 is holding me back far more than that 6870.