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buy applecare...

He is correct that you have one year from the purchase date to add AppleCare... just make sure you don't forget. It is worth every penny.
Amazon usually has it a little cheaper, and sometimes you can save as much as $50. Buy your ram third party if you are trying to save some money...
I just bought 16gb ram from RamJet for just under $500, and OWC was about the same (Apple is $1000).
cheers,
michael
 
This is what I did...

I ended up getting the machine with the stock memory and bought Applecare the same day.

Then because my birthday was 3 days after I got the machine my in-laws gave me a check (they can never tell what I want but it was a good thing really) as well as I sold my old iPhone I ended up getting 8 gigs of ram from NewEgg for about $214 (a screaming* deal I must say).

Now I've got a machine with 12 gigs of ram and 3 years of Applecare and no problems thus far.
 
AppleCare you can get later on from aftermarket but the CPU and GPU are not easily upgradeable. Get the 2.8GHz i5 as it comes with 1GB GPU
 
It would certainly help to know what you're planning to do with the computer. Without knowing your usage specifically, I'd say get the graphics card. It'll likely make more of a difference in the future (I'm thinking Steam here...) than the dual core i5 vs the stock i3. It is a $150 upgrade, so if you can swing the $300 to get the quad i5 it comes with the Radeon 5750 as well.
 
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