Under C# is Int64 use on a 32 bit processor dangerous

Even if the writes were atomic, chances are you would still need to take out a lock whenever you accessed the variable. If you didn't do that, you'd at least have to make the variable volatile to make sure that all threads saw the new value the next time they read the variable (which is almost always what you want). That lets you do atomic, volatile sets - but as soon as you want to do anything more interesting, such as adding 5 to it, you'd be back to locking.

Lock free programming is very, very hard to get right. You need to know exactly what you're doing, and keep the complexity to as small a piece of code as possible. Personally, I rarely even try to attempt it other than for very well known patterns such as using a static initializer to initialize a collection and then reading from the collection without locking.

Using the Interlocked class can help in some situations, but it's almost always a lot easier to just take out a lock. Uncontested locks are "pretty cheap" (admittedly they get expensive with more cores, but so does everything) - don't mess around with lock-free code until you've got good evidence that it's actually going to make a significant difference.

Is this really true? Yes, as it turns out. If your registers only have 32 bits in them, and you need to store a 64-bit value to some memory location, it's going to take two load operations and two store operations. If your process gets interrupted by another process between these two load/stores, the other process might corrupt half your data! Strange but true. This has been a problem on every processor ever built - if your datatype is longer than your registers, you will have concurrency issues.

Is this something I would worry about in the real world? Yes and no. Since almost all modern programming is given its own address space, you will only need to worry about this if you're doing multi-threaded programming.

If my application is multi-threaded do I really need to surround all my Int64 assignments with locking code? Sadly, yes if you want to get technical. It's usually easier in practice to use a Mutex or Semaphore around larger code blocks than to lock every individual set statement on globally accessible variables.