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.