Just plug in new card?
The 380 requires 150% of the power of the 4850.
ok the pins from the power supply ,that go into both cards the same?
It will depend on the card. I have seen them needing 2 6 pin connectors or like mine a single 8 pin connector.
Well technically speaking "yes" you'd unplug the old card and plug in the new one (also the 380 will require 1 more 6pin PCI-E power connector or possibly 8 pin depending on the model) . Uninstall your 4850 drivers , swap cards, install new drivers and profit.
However Like bz pointed out if the power supply isn't powerful enough when that 380 fire's up. either A if its a good PSU it'll hit overvolt protection and your entire PC will turn off.
Or b. it'll exploded in a spectacular fashion
or C. It'll exploded and take more then just itself with it.
As a side note they typically really won't explode but usually a pop and bang followed by lots of smoke and maybe a little fire.
So please be smart and check the total 12V rail power output on your power supply. before spending the money on a 380. if you are insure feel free to respond.
Lastly just make sure your psu got the additional 6+2 connector for the card - my R9 380 needs the 8pin to be able to run in PCIE3 x16 @ 16.x mode. The 380 is quite a hotplate hence make sure there is enuf air through the case to keep everything cool especially in summer.
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