Okay, so it's down to how Second Life progresses. I also see evidence of that with my, horribly old, 8600GT. It ran Second Life on Ultra settings 2 years ago with no issues.
After some research, I have now figured that I can afford the 560 Ti. But, would this last me only 2 years roughly still?
I could see myself upgrading again in 3 years - maybe. But 2 years would be too soon.
I too added a 600w power supply to this system. So, it's future proof in that sense at least. The 680 is nearly £400 here in the UK. I'm certainly not in a position to pay that.