AFAIK The -d64 and -d32 only ever worked on Sparc Solaris.

If you have 64-bit JVM it will run all your programs unless they use a 32-bit shared library, in which case I suggest either a) don't do that or b) use the 32-bit JVM only when you need to by specifying the path.

Per java.com, you only need the 64bit JRE if you plan on running a 64bit browser. Windows x64 systems by default have seperate 32 and 64bit versions of IE, so if you plan to use the x64 option (x86 is default), you need a 64bit plugin and JRE.

If you plan to run Firefox/chrome/whatever-other-x86-browser, just use the 32bit.

if you are getting a JDK for development determine what version is appropriate for your IDE or other development tools, and use it.

No, there are no official portable zipped distributions, but you can make one yourself, it's quite simple. Here's an instruction on how to do it for JDK 7 (for JDK 8 the procedure should not be different):

http://www.brucalipto.org/java/how-to-create-a-portable-jdk-1-dot-7-on-windows/

In a nutshell, you should unpack the .exe file as a 7-zip archive and rename all of the *.pack.jar files inside to corresponding *.jar files (just remove ".pack" from their names).

64-bit vs. 32-bit really boils down to the size of object references, not the size of numbers.

In 32-bit mode, references are four bytes, allowing the JVM to uniquely address 2^32 bytes of memory. This is the reason 32-bit JVMs are limited to a maximum heap size of 4GB (in reality, the limit is smaller due to other JVM and OS overhead, and differs depending on the OS).

In 64-bit mode, references are (surprise) eight bytes, allowing the JVM to uniquely address 2^64 bytes of memory, which should be enough for anybody. JVM heap sizes (specified with -Xmx) in 64-bit mode can be huge.

But 64-bit mode comes with a cost: references are double the size, increasing memory consumption. This is why Oracle introduced "Compressed oops". With compressed oops enabled (which I believe is now the default), object references are shrunk to four bytes, with the caveat that the heap is limited to four billion objects (and 32GB Xmx). Compressed oops are not free: there is a small computational cost to achieve this big reduction in memory consumption.

As a personal preference, I always run the 64-bit JVM at home. The CPU is x64 capable, the OS is too, so I like the JVM to run in 64-bit mode as well.