In order to answer your question, you really need to list what you use your system for the most. The e series sandy bridge came out the other day at newegg, but it was $600. I wouldn't invest so much in one cpu unless the benchmarks showed at least a 50% increase. Since I don't work much anymore, I wait for all the poor smucks who can't wait to spend more $$ on their next system and part out their old components. I let them take the hit on depreciation. Here's a link to some benchmarks for the e series: http://www.tweaktown.com/news/21543/leaked_sandy_bridge....
Ivy Bridge will most likely not offer much performance boost over Sandy Bridge. Estimates generally range between 6% - 10%. I'm guessing 6% because the CPU architecture is already pretty mature and Intel is focused on their next gen CPU; Haswell. It's too early to tell how much of a performance boost Haswell will have over Ivy Bridge or Sandy Bridge.
There has never been a family of CPUs that boosted performance by 20% - 30% over the previous generation. It is doubtful that even Haswell could pull off such a feat, not to mention Ivy Bridge.
Which would be best for future proofing and best value? and Why?
Unless you consider yourself an enthusiast willing to spend any amount of money for the best performance possible, then Ivy Bridge is the better choice. Performance will probably be 6% - 10% better than Sandy Bridge (at the same clockspeed), but it potentially it should be able to achieve higher clock speed due to less power and heat as a result of the die shrink.
If all you do is play games, then you will not need a CPU with more than 4 cores because games can only use up to 4 cores. From an Anandtech review about CPU scaling in games that can use up to 4 cores; the performance boost from 2 cores to 3 cores averaged around 27%, the performance boost from 3 cores to 4 cores averaged around 6%. The 5th, 6th, 7th, 8th, etc... cores will provide 0% performance increase.
If on the other hand you do video encode, 3D modelling and rendering, image editing, and other highly intensive multi-threaded applications, then you can see a benefit from more than just 4 cores. Assuming those programs support more than 4 cores.
As gmaster456 said and predicted. I am currently do not need the power right away. I am using my computer for rendering 3dsmax and other 3d programs. I occasionally play game and which I was surprise and learned from this feedback, that games do not excel 4 cores.
Thank you all and made me understand clearer.
Does anyone happen to know the price for Ivybridge?