I heard that some 5600 xt have been updated to be faster. Which 5600 XT should I buy?
I will wait until you could find models with boost clock of 1,750MHz with vBIOS applied by manufacturer.
None of the models listed online show boost clocks above 1,640MHz,
Manufacturers like ASRock, Gigabyte, XFX, etc have BIOS updates online to unlock the 1,750MHz boost clock.
You will have to re-install the drivers after the vBIOS update.