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The only thing you should really think about is what the new price points and standard configurations will be. Along with the new GPUs that you'll have in the newest model.
 
I'm more interested in the addition of USB 3.0 than in Sandy Bridge. If Sandy Bridge was out, I'd be happy to have it, but if not, I'd be happy with an i7 2.9GHz now too.
However, I'm not happy with the speed of USB 2.0. My iPhone, iPod, iPad all takes too long to sync. Backup drive, flash drive, takes too long to copy. There will always be a little faster processor every year. But USB takes many years to change. I built a PC with USB 3.0 last April. So I hope to expect it in the next iMac this year (and in iPhone 5, iPad2, etc.). I would be happy with the current i7 CPU for the next 3-5 years, but as for USB 2.0, I already need more now. That's keeping me from pulling the trigger yet on a newer Mac for work, so that I'm not stuck with USB 2.0 for the next 3-5 years..

I think you will find that having a system with USB3.0 will have no effect on how fast you sync/backup to iPhones and iPads. I could be wrong but those devices only transfer at USB2.0 speeds.
 
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