Once you get this working it would be interesting if you write another post about whether or not your working speed in Photoshop has actually increased. I used Win64 for a time with 2gbs RAM, but I was not convinced that working speeds were significantly faster than with 32bit Windows. I had to give up on Win64 because of the many, many problems with this OS: I wish you luck, a reliable anti-virus program and a reliable CD/DVD writing program.
The Tech Doc DOES address this:
When you run Photoshop CS2 on a computer with a 64-bit processor (such as a G5, Intel Xeon processor with EM64T, AMD Athlon 64, or Opteron processor), and running a 64-bit version of the operating system (Mac OS v10.3 or higher, Windows XP Professional x64 Edition), that has 4 GB or more of RAM, Photoshop will use 3 GB for it’s image data. You can see the actual amount of RAM Photoshop can use in the Maximum Used By Photoshop number when you set the Maximum Used by Photoshop slider in the Memory & Image Cache preference to 100%. The RAM above the 100% used by Photoshop, *which is from approximately 3 GB to 3.7 GB*, can be used directly by Photoshop plug-ins (some plug-ins need large chunks of contiguous RAM), filters, actions, etc.
If you have more than 4 GB (to 6 GB (Windows) or 8 GB (Mac OS)), the RAM above 4 GB is used by the operating system as a cache for the Photoshop scratch disk data. Data that previously was written directly to the hard disk by Photoshop, is now cached in this high RAM before being written to the hard disk by the operating system. If you are working with files large enough to take advantage of these extra 2 GB of RAM, the RAM cache can speed performance of Photoshop