I run both 64 bit and 32 bit PostgreSQL 9.0 RC1 beta on 64 Bit Vista and 64 bit Windows 7 for development. There are 64 bit libraries like libxml 2 and dependents that need porting to 64 bit Windows.

In general I can run both versions of PostgreSQL concurrently, on different ports, on the same 64 bit box and connect from pgadmin III.

The 64 bit Windows O/S has a stricter security model so I generally write code in 32 bit and test there before deploying to 64 bit for testing.

The Windows 64 Bit O/S runs 32 bit apps run under WOW64 emulation with thunking for system calls. If you are planning to run 32 bit production apps on a 64 bit box check out the MSDN site for the performance and memory limitations of WOW64.