Ok, if ya wanna nit-pick.... Like all things in reality (unlike the un-reality of theoretical mathematics-- ever seen a perfect circle? Ever calculated Pi?), things change and there are no constants, no guarantees; even the supposed "law" of gravity became obsolete with Einsteinian relativity (you can argue the obsolete part, but the fact is that newtonian gravitation doesn't fully describe reality regarding gravity's actual behaviour/effects; i.e., the reality that the classical law of gravity describes IS NOT the reality we live in). The fact of the matter is that despite how hard we try, science is a TOOL that DESCRIBES reality. We can only ever strive to approach this theoretical absolutism through statistics/empiricism. So you say that flipping a coin gives you a 50% chance of getting heads? Prove it. Answer: impossible. The key is that one has BOTH the theoretical aspect and the statistical aspect, the former perhaps guiding the latter, while the latter refines the former with an increasingly accurate description of reality. Long story short, if you look at a graph of the history of moore's supposed law, it's pretty dang close. Based on my own research, the figure seems to stay very close to this 1.5x rate going from the 7800 series up to the gtx 280.
And if it'll put you at ease (b/c I just know you're afflicted with consumer remorse! ), I'll tell you that if you stay at a certain price tier, like the gtx 280 that you have, from one generation to the next, you are always still getting this 1.5x rate, even if you spend $500 each time (1.5:1 ; no increase in price)--- so it's NOT a technically horrible value; again, it's only a really bad value when you spend MORE and get less than this 1.5x rate of return (which will happen going from, say, a 4830 up to a 4850) --going from one price tier up to another; so, basically it's bad that you got there, but since you're there it's not so bad to stay there according to this one aspect of the method. OF COURSE, when you combine this with the simple fact that you get more for your dollar at the mid-range tier... So I'm going to get that same nice proportional boost at 1/8 the price But again, needs come into play, I only play at 1440x900, so the rez isn't too challenging; that's the funny thing with monitors, they might impact the P/P more than any graphics card choice you'll ever make in a way.