I am sorry I did not provide the advice you needed.
But honestly, if all you needed to accomplish was Web Surfing, any of today's Macs are capable of doing that and way more -- including AirFrame Stress Analysis and Fluid Dynamics modeling. We used to require Barn-sized mainframe class computers to do that work.
Now that you indicate that you need to run Final Cut, then the 2009 and later Mac Pro is your best bet, in my opinion. It will accept larger RAM memories, and its memory is Error-Correcting Memory, so you will not face the problem of bad memory causing undetected grief. It has PCIe expansion slots, so SSD-on-a card is an additional speedy option, as is eSATA external expansion.
If you buy a fast four-core model, it can be user-upgraded in the future to the 6-core. Any 6-cores that appeared on the Refurbished list were snatched up in a hurry. The slowest 4-core will tend to be a bit slower overall, because some tasks are still not multi-threaded enough to make a big difference.
The 8-core pays a penalty for its two-chip solution. Coordination between the two chips takes a lot of overhead, and you lose some of the increased power you hoped for. And the prices of those machines mean you could almost have two complete four-cores for the same price.
The 2013 model has taken a strong step into the Professionals-only category. Its raw compute power (including better OpenCL processing power for array processing) and graphics ability are remarkable. But its price point (including required expansion chassis) is so high that only those using it all day every day to make money can justify the cost.
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I find the iMac less than compelling because it is not upgradeable in more than its memory size, and that has a limit too close to what one might like for Final Cut work. I also worry that the screen and the processing-unit will age at different rates, and one will be obsolete long before the other.