Using a 64 bit version of any OS doesn't make it any more immune to malware. The 64 really only refers to the rate at which data is ****** and processed by your hardware.
x64 is not faster per say... That is controlled by your hardware. x64 allows a wider bandwidth of data, and if an application is written in 64bit the os allows the hardware to take advantage of this. Few aps are available yet, so in practice x64 is not any faster than x86...
x64 will not, and cannot run 16bit aps... many malware installers are 16bit, if an address tries to run one, my machine hangs and tells me a script is not working correctly, and asks if I want to continue running it... just say no and problem solved. There are a couple installers that are newer and do run, but malwarebytes takes them off.
The script hang thing happens often, I run a scanner maybe once every couple of months. That's with no firewall, no security on my router, etc...