You will not be able to upgrade to a 2nd or 3rd generation CPU. Assuming all three generations uses the same socket, the BIOS will not recognize the newer CPU. If the BIOS cannot recognize the CPU, then the laptop is effectively dead. Dell would need to release a BIOS that would allow the laptop to use newer gen CPUs. It is not in Dell's or any other laptop manufacturer best interest to let you upgrade the CPU beyond a certain point. They much prefer that you buy a new laptop.
The best CPU you can install is basically the most powerful that was offered for that specific laptop model. If the i7 920XM was never offered as a CPU option for your laptop, then it is possible that the BIOS will not recognize it.