You'll need:
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Latest BREED bootloader for Mi WiFi R3G (I don't test on the original bootloader, since I don't have TSOP48 Programmer, I don't want to risk anything)
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A 512MB 1866 MHz DDR3 1.5v (not DDR3L) chip, I used mine from an android box that I have lying around that have 4 of that chip
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A hot air solder station to take out the old RAM chip and put the new one, if you don't have one, maybe you can ask around the local phone/laptop hardware repair shop to do it for you.
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Luck, lots and lots of luck
As for the OpenWrt side, you still can use the official images, it will detect the new RAM size automatically, no change needed.