I have installed R14 on an XP_64 laptop with no problem so I guess R15 would run on it as well.
No noticable advantage going to 64bit though however getting the drivers for XP_64 was a pain. Still looking for the WLAN and modem
I'd not suggest going to 64 bit yet, stick to normal windows, we had problems with some other programs not installing (XP 64 can only process 64 bit and 32 bit data, where as XP 32bit and process only 32 and 16 bit processes). Unfortunatley there are loads for installer programs that are 16 bit, so although the program should run ok, it's not possible to install.
I noticed that with r15 (and 14.5, for that matter) I'm able to get a single CATIA session to use 3GB of physical memory if I'm on Windows XPx64. I topped out at about 2.5GB on Windows XP.
R15 in 64 bit only on AIX. And only in DMU, FEA, and Drafting workbenches. And VB won't work in 64 bit. And VPM won't either.
It will use tremendous amounts of memory (we tried it with 8GB). We were able to load an entire aircraft (~18K parts), but the number of triangles made the graphics card the new issue.
R16 is supposed to be 64 bit on windows and all workbenches. Don't know yet about any restrictions, however.