Advantages of booting my Mac into 64bit?

Here is a good article about the advantages of having 64 bit ram: http://arstechnica.com/apple/reviews/2009/08/mac-os-x-10-6.ars/5 and it covers more than just ram

If you only have 3GB of RAM you should just try both and decide which you really think feels better. 3GB will be recognized in a 32 bit OS as well as a 64 bit OS. So there is really nothing being lost when using just a 32 bit version.

If you plan on upgrading to 4+ GB you will need to use a 64 bit version though.

Haven't we answered this before? 32-bit OSes can still recognise more than 4GB of RAM. The benefit is that the bus is 64 bits wide, and some of the registers are 64 bits wide. It's mostly a marketting gimmick, because programs still need to be specifically written and compiled for the hardware you are using to run at it's most optimal.

So, what about RAM? Well, there's PAE, but that involves sending 36 bits over a 32-bit bus, doesn't it? Are either busses limited? Well, is binary limited because it has 2 digits rather than 10? Can your car not be used to transport 100 people, 3 at a time to a different location? First advantage: The 64-bit bus ends up being able to send more data over the bus at once.

So, what about the registers? Well, let us assume you have a 32 bit register, each bit can be a 0 or a 1 (that's 2 digits) so you have 00000000 00000000 00000000 00000000 through to 11111111 11111111 11111111 11111111. In decimal (base 10) that's 0 through to 4294967295 (2 ^ 32 - 1). Does that mean our computers can only count from 0 through to 4294967295? Well, what happens once we run out of values that we can represent with 1 digit in decimal (eg. add 1 to 9)? We move on to 2 digits. 32-bit programs can do the same: They can use multiple 32-bit registers to store a single 64-bit value. However, the 64-bit register in a 64-bit processor can add, subtract, multiply, divide, etc with single operations. The 32-bit alternative can't, so the second advantage: 64-bit processor is faster at working with 64-bit numbers (from 0 to somewhere around 18446744073709552000, if you ever need numbers that large).

So what's the advantage of booting an OS that knows about 64-bit registers and a 64-bit bus? Well, it should be clear that it's faster. You already knew that. I hope this was helpful ;)