Now Ryzen 7900X is cheaper than 7900 what is weird. Is it possible to run 7900X undervolted to reach the low power consumption of Ryzen 7900? If yes, is it better to buy this cheaper 7900X or 7900 which has 65W TDP?
It's better for games, better performance. The cache makes a big difference. And has better cache/core distribution than the 7950x3d and 7900x3d, so it has consistently better performance for games.
The 7900 is only better for tasks that can take advantage of the extra cores, which are basically 0 for normal users.