Switch on Enhanced Protected Mode (Internet Options > Advanced tab > Security section) and your tabs should switch to 64-bit. The full mind-numbing explanation is here if desired:
http://blogs.msdn.com/b/ieinternals...rk-security-addons-cookies-metro-desktop.aspx
In a nutshell, when you have 64-bit Windows, enabling EPM will enable 64-bit tab processes, and reportedly high-entropy ASLR too (a good thing). If it's 64-bit Windows
8, then enabling EPM on the Desktop version of IE10 will also put tabs in AppContainers to further isolate damage if one of them gets pwned.
I've been messing with the IE10 preview on Win7 at home, with EPM enabled. So far I mostly like it. One downside is that since EPM restricts the browser's ability to write to the file system more, I run into an error if I try to right-click a picture and save it.