Work is being done for the next version, and most likely PostgreSQL 8.5 will run as a native 64-bit binary on Win64. It remains to be seen how many of the third party pieces will work (for example, TCL and MIT Kerberos don't currently provide 64-bit versions on Windows), but the core database should be available.
Note that this would really only be necessary if you want either total work_mem or shared_buffers to be very large. In most cases, that won't be a problem, but if you are running large data-warehouse style queries for example, it might be interesting even with as little memory as your server. But it's mainly being developed to deal with large memory systems, and compatibility with third party libraries.
That said, PostgreSQL will run faster on a Linux/Unix based platform, so if you have that as an option, you should go with it. PostgreSQL has been designed for a Unix architecture, and keeps this architecture on Windows (for example, processes rather than threads), which makes it slower there.
As for which distribution you choose, it doesn't matter from a PostgreSQL perspective. Pick something that your administrators, or someone you work with, feel comfortable with.
Oh, and the proper URL for the blog post referred in Andrews answer is http://blog.hagander.net/archives/73-PostgreSQL-vs-64-bit-windows.html, and it contains an explanation on the memory issue with 32-bit PostgreSQL on 64-bit Windows.