I know very few people who upgrade their CPU/mobo every year, or even every two years. Many people are still gaming with Core 2 Duo systems with Radeon 5xxx/GeForce 4xx series GPUs and are quite happy. Personally I plan on keeping my i7-930 system as my gaming system through at least the successor to Ivy Bridge, and honestly beyond. I've got a budget to keep and part of that is not spending $1500 every year building the best mid-high end gaming machine I can.
Don't just look at the maximum potential and say 'I've got to have it', look at the data people have provideded you and be informed.
The point when it would become 'needed' is when you have a game you are playing and it's not performing how you like. After you change/OC your GPU around to boost performance and it just doesn't cut it, and you've OC'd your processor to the max, that's when I would start shopping around. Even then I'd probably still just set game details to medium first and live with that for awhile before upgrading.
As has been said here by everyone else so far, it would be an excessive waste of money to upgrade what you have now to a 4 core Sandy Bridge.
Because you have an i7 930 rather than an i7 980x, I'll assume you don't go make crazy computer purchases just for the hell of it.
It will probably be at least 3 years before your CPU becomes a bottleneck to being able to play games, so I would be looking to do a GPU upgrade(if you are a gamer) when ATI and Nvidia have 28nm cards out, if there are any decent games out that would benefit.
I know with my i7 860, I won't be doing a CPU/motherboard update till Haswell generation at the earliest and even then I think I would rather get the 16nm successor.
Hell if AMD surprise on the IPC and performance front, my next CPU may once again be an AMD powered one, but either way, not for 3 more years for me.
Yes my i7-930 was more than I needed when I bought it and will be for some time, but that doesn't mean I want to go buy a SB setup, then a Ivy Bridge, then a Rockwell, Haswell, etc. Do I have the money? Maybe, but there are plenty of much more worthwhile things I can do with my money (charity, help friends out, etc.).
For some it is a way to get their kicks to always stay on top, but for me it's just fun to be able to participate at a relatively high level if only for a short time every few generations.