64-bit Windows (all versions) supports both 32-bit and 64-bit executables and libraries.
The bitness of the ODBC consuming application dictates the bitness of the drivers it can use. 32-bit IIS requires 32-bit ODBC drivers, 32-bit Client SDK. 64-bit IIS can be set to run in 32-bit mode, in which case it requires 32-bit ODBC drivers, 32-bit Client SDK. In default 64-bit mode, 64-bit IIS requires 64-bit ODBC drivers, 64-bit Client SDK.
32-bit ODBC Drivers are configured with the 32-bit ODBC Administrator. On 64-bit Windows, this is
C:\Windows\SysWoW64\odbcad32.exe. On 32-bit Windows, this is
64-bit ODBC Drivers are configured with the 64-bit ODBC Administrator. On 64-bit Windows, this is
C:\Windows\System32\odbcad32.exe. On 32-bit Windows, this does not exist.
System DSNs will be your best choice in almost all cases. User DSNs are rarely the best choice.
Generally speaking, you want the latest version of the Informix CSDK regardless of the version of the target Informix instance. Given that the target Informix instance is not on the same machine as the CSDK, the bitness of the CSDK and the target instance may but need not match.