There seems to be a huge amount of misinformation and misunderstanding about the real-world advantages of a 64-bit CPU. I believe I have a reasonably good idea of what the facts are, but I can't seem to find a decent, accurately-written FAQ on the subject anywhere. Here is one FAQ on the subject that, while it's rather technical, does provide a reasonably good idea of the issues involved:
http://www.anandtech.com/guides/viewfaq.html?i=112
Basically, a 64-bit processor lets you do two things that a 32-bit one doesn't: Make use of more than 4GB of RAM, and process 64-bit data chunks.
The RAM issue is a biggie for high-end pro applications and especially servers, but really has little to do with most regular PC users or gamers--a couple of gigs of RAM is more than enough, even for high end gaming.
The 64-bit intiger issue sounds good--twice as much data=twice as much speed--but it's really not that simple. The thing is, the chunks of data that are passed around in most applications don't need to be any bigger than 32-bits, and regardless of the upper limit to the size of a chunk that can be processed, the CPU can only process one chunk at a time.
That means that, basically, most applications get no benefit whatsoever out of a 64-bit CPU--they don't need either the extra memory or the bigger data chunk size. I'll also mention that the G4/G5's 128-bit Altivec unit CAN process two 64-bit or four 32-bit chunks in parallel, which is why it's so fast on operations that can be heavily optimized for this kind of processing.
If you're a scientist or running a server that does deal with big chunks of data and lots of RAM, then 64-bit kicks butt, though.
All that said, games actually will see some benefits eventually, I believe. Though a lot of a game's speed is based on the graphics card, there is still quite a bit of work in most games that is done by the CPU, and the kind of work that CPUs do might in fact benefit at least a little by being 64-bit--I've heard things like more detailed textures mentioned (though probably not actual "performance" increases--just better looking graphics).
A 64-bit version of Unreal Tournament 2003 exists as a sort of technology demo for the Athlon 64, although I've never seen a comparitive benchmark for it. I asusme that, eventually, there will be a few high-end games that give you a bit of extra graphics quality on a 64-bit CPU, but likely not many for a while, and even longer before we see any on the Mac.