To decide if 64-bit computing is "really all that" you have to understand what it is.
64-bit computing by itself ONLY means that the General Purpose Registers are 64-bits wide. That means increased dynamic range. Using the binary number system, a 2-bit processor gives you 4 possible values, a 3-bit gives you 8, 4-bit gives you 16... and a 32-bit processor gives you 4,294,967,296 possible values. (sounds kinda close to the 4 GB RAM limit of 32-bit processors doesn't it? ) That is it's dynamic range. A 64-bit processor's dynamic range is approximately 4.3 billion times greater than a 32-bit processor... which simply means, it can work with much larger numbers. Why is that important? It's not really to you or I at this point... but it is in applications like weather simulators and such.
64-bit computing also allows for more RAM than a 32-bit processor because of it's increased dynamic range. Because as I just figured, a 32-bit processor can only handle about 4.3 billion values, which translates to about 4 GB of memory. A 64-bit processor has an upper limit of about 18 million terabytes... (32-bit = 0.0043 terabytes... 64-bit = 18,000,000 terabytes).
That's speaking ONLY in terms of 64-bits vs. 32-bit... that has nothing to do with the architecture of the specific processors. AMD changed some more things when they designed the Athlon-64.
First, they used a 40-bit memory address rather than 64-bit since we're not going to need 18 million terabytes of memory anytime soon... the 40-bit address allows up to 1 terabyte of memory.
Second, they doubled the amount of General Purpose Registers so there's 16 now rather than 8. So not only are they twice as big, there's twice as many. But they can only be used by 64-bit software, so the benefit of extra registers isn't realized with 32-bit software.
Third, they lengthened the pipeline by a few stages. What that boils down to is basically higher clock speeds are easier to reach.
Fourth, they built the memory controller into the core, which eliminates almost all latency issues with the memory controller. Basically the memory is now just connected to the CPU by wires, whereas with the Athlon XP and Pentium 4 for example, the CPU was connected to the northbridge chipset on the motherboard, and so was the RAM. So the northbridge chipset sat between the RAM and the CPU.
Fifth, they added support for SSE2, so applications designed to take advantage of Intel's SSE2 instructions can now also take advantage of those instructions on an Athlon-64.
Sixth, they're using SOI, which in a nutshell, reduces current leakage within the processor, which allows the millions of transistors to switch on and off as fast, or faster with less voltage, which means faster speeds and less power consumption.
They've made other changes as well... but those six are the main ones that will effect performance.
So is 64-bits "really all that?" That depends on what you consider to be "all that." It's important to remember that all Athlon 64's are 64-bit processors... but not all 64-bit processors are Athlon 64's. So when you ask a question like this, you have to be specific... are you talking about 64-bit computing in general? Or are you talking about currently available 64-bit desktop processors (Athlon 64's)? If you're talking about 64-bit computing in general then no, it's not that big a deal because we're not really pushing the limits of a 32-bit processor's dynamic range other than in large servers that need lots of memory, but Intel has found a way around that without increasing the dynamic range that allows them to address more than 4 GB of memory.
So to answer your question... IMHO, yes... 64-bit (more specifically, x86-64) is "really all that" ... and a bag of chips, lol. Mainly because of the larger amount of memory it's able to address natively, and also because of the way currently available 64-bit processors are designed, with twice as many GPR's and an on die memory controller. The benefit of most of these features remain to be seen as of right now... although we have a pretty good idea from the beta software that's being played with and reported on.