Omaro :
what is the max GPU VRam that optiplex 3020 can support if we upgraded the psu & cpu to i7-4790k as its FCLGA1150 ?
Edit: keeping in mind case size and if we changed the case
it is an irrelevant question, as cpu and psu have nothing to do with vram amount
the question you should be asking is:
what is the best gpu you can get for your budget that will work with your power supply and fit into your case
you will definately need to upgrade the power supply
and you are limited to cards under 8 inches in length
so before anyone can answer, what is your total budget?