I'm not running Vista 64 but XP64 and I have to agree, the huge advantage is to be able to use as much memory as your motherboard can get (not to mention your bank account ;))
I currently have 8 go (which is, I agree overkill, but I got lucky on ebay and  I got the 8  go for just slightly more than I would have paid  for 4 so .... :)  Being able to run  Vue or poser + photoshop  + my  modeling program without having to close one before opening the other is really something I greatly appreciate, especially when fine tunning a texture. Being able to render a scene that will use 4 go of memory in my main 3D application is also a big plus. No more worry about too much maps (it doesn't mean I don't try anymore to optimize my work, but I won't leave out a needed specular map because I hit the 2 go upper limit)
Other than that, I have to agree, no huge speed improvements, sometimes you gain a few % but well, 2 minutes on a 20 minutes render, you don't really notice it (maybe different for those who do animations though). Off course, this also mean a lot less 3D application crash (most of the time, max crashed when rendering for lack of memory, nothing else)
I'm not ready for Vista, and I guess I won't until I have no real choice (nothing to do with the "Vista bashing trend", it's just due to the fact I'm not really interested by what Vista would bring me - as an example, I run my XP 64 with the classic interface, the Win 2K look :)) but going 64 bit is definitely a big plus when you do some serious 3D and you want the limit to be your skill and not your hardware (I guess when it comes to software, whatever 3D application you use, unless you're exceptionally good, it's hard tor reach the limit anyway :) )

J-L