Do you really need a new case? That shouldn't be contributing to crashes and if you reuse the old one you can put the money you save towards the 16 GB 9060 XT instead.
Otherwise you could save about half the money for upgrading to the 16 GB 9060 XT by swapping to a ThermalRight air cooler (Peerless Assassin or Phantom Spirit), the 7600X isn't nearly demanding enough to need liquid cooling. You could then opt for a cheaper case if necessary to fit the 16 GB 9060 XT in your budget.
Bottom line: Do what you can to get the 16 GB 9060 XT.