virtualbox.org

Running VB on Leopard host. Wondered if there's a difference both in terms of guest performance and load on the host between Ubuntu 32 bit and Ubuntu 64 bit as guest? I'd obviously prefer the one with the best performance and lowest load on the host

You must have VT-x if you want to run the 64 bit Ubuntu anyway. But if you can, there isn't really a 'best' version. I haven't noticed any difference in performance on my system between the 32 and 64 bit version.