I think if you're happy with you're current setup then upgrading, while fun, wouldn't be the sensible thing to do. Still, you could argue that you're "future prooofing" things for that little bit longer lol. Still, if you're asking the question are you really satified with your current performance levels?
I had a single GTX 570, running pretty much everything at high or highest settings @ 1920x1200 just fine. Really, I was very happy with my lot. Then a 2nd 570 came up cheap 2nd hand on these very forums & in a bored moment (and wanting something new to play with) I took the leap to SLI. The transformation was amazing! Games that ran at a "perfect" frame rate (or so I thought) suddenly became silky smooth. Trying to measure the difference using MSI Afterburner I saw that I'd previously been running at a strong 40fps or so in the titles my card had to work harder on. Now I was seeing a vSync-limited 60fps! I was really really surprised by the difference - plus I had none of the 101 issue people said I'd get going to SLI.
Now, for some people - a good friend of mine being the prime example - there's NO difference between 40 and 60 fps - he just cannot see the difference. To me 40 and 60 are worlds apart, especially in something like Skyrim or Crysis 2 - or a number of other titles I play.
I confess, when the 680 came out I considered getting a pair, even though my current gamer eats everything up at 1920x1200. Then the 670 came out at a good price point vs. the 680 - basically over £100 less for at least 90% of the performance. Again I was tempted but, really, what would be improving?
For me, the "sensible" time to upgrade is when you cannot play what you want as smoothly as you expect - that's assuming you're not happy turning the odd setting down initially. Also, sometimes it's nice to upgrade for a new feature, such as DX10 (fairly pointless that one) or DX11 when you have a card that supports an older standard. For me I already had the latest DX11 tech, so I doubled up. The joy of the SLI/Crossfire upgrade path is often you double your GPU's for far less than double the cost. However, I think I'll only make that saving the one time as for my next rig I fully plan to go SLI from the go - I'm that impressed with it.
Anyway, I ramble as usual - what what the question again? Lol.
Scoob.