Up until now, the unofficial Google Chrome OS for Macs, Chromium, has only been available in a 32-bit download, but today you can grab the ChromiumOS64 if you're more RAMmed-up.
Teo en Ming, the ChromiumOS64 project lead, says of the download:
"The original Chromium OS project does not support 64-bit platforms. This is because the majority of the netbooks in the market are based on Intel Atom processors, which are mostly 32-bit."
With Chrome OS launching properly at the tail-end of 2010, we've got a full year to wait before we see anything more solid become available. And even then, it'll only be available on Google-branded devices, with Acer hoping to launch the first official netbook running it.