I can't see how the 780 makes such a difference over the 775, is this for photoshop or video?
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It's exactly the same a options in a car, do you save the money initially and go with 'basic' options and therefore a lower resale value plus being harder to move on. Or do you spend the money and make it easier to sell come upgrade time.
Apart from that for photoshop, the Difference between a 775 and a 780 is not worth spending the extra bucks on as both cards will be blisteringly fast in a comparable machine.
The only benefit of the 4GB card is for demanding games (which you say you're not interested in on the Mac) and resale. That is it.
It's actually always easier to sell the lower cost base model than it is to unload a higher price upgraded model.
With Apple it works like this: the $1099 model will sell for $800 used, and the $1499 model will sell for maybe $900.
Why does it work that way?
#1 Most people who are buying second hand aren't after the latest and greatest and are looking for a bargain, and therefore, upgrades have less value to them.
#2 Because people buying 2nd hand tend to be budget limited (hence why they don't want to buy new) its a lot easier to find people willing to pay the base price than it is people who are willing to shell out big bucks for a 2nd hand unit. I have had this happen to me. When I sell a base model it moves fast, can usually dump within 24-48 hours and when I try to sell a high end unit I either have to lower the price to move it or it takes weeks or longer to find potential buyers (and even then only at a steep discount).
#3 When its time to sell, the differences are trivial because both models are going to be outdated and the new stuff is much better. So while right now its worth shelling out money for the better video card (if you need it), the two cards will be nearly indistinguishable from each other once compared to the new stuff that will come out in a couple of years.
I have been buying and selling Apple stuff for about 7 years now and have seen these dynamics played out. My conclusion: if you are going to get benefit from the upgrade then go for it. If you are upgrading only to "future proof" or "raise resale value" you are wasting your money.
Computer parts are
never an investment. And an upgrade will never even pay for itself during a resale situation, much less help you profit from it.