Upon PXE boot, the only options you have is to boot into any current or default 32bit images. This happens even if 64bit images are loaded in the WDS console as an install, boot or capture image and have no known problems.
To rectify this and enable client architecture detection, just run the following command from an administrator console on the server:
wdsutil /set-server /architecturediscovery:yes
The client will boot as normal but download a small program called wdsnbp.com which will determine the client architecture type and allow selection of 64bit WinPE as long as the client supports it.