I use windows 7 64 bit.
Usually I use opera as browser.
However I keep IE8 64bit as sometimes some places need IE.
Reading your question,I started IE8 64 bit browser, and on the bottom appeared
Do you want to make IE8 your default browser?
I said yes, and it works ok for me.
And it definitely is the 64 bit version, as I removed the 32 bit one
All the best