Can you be more specific? Have you actually downloaded the jre and tried to install it?
I notice that Sun's download page does not have an x86-64 version of jdk1.5 for windows (there is one for both linux and solaris). That probably means that nothing from sun will work for you.
Bea has a 64 bit windows jdk. It's listed as xeon 64 bit but that should be fine on amd as well, in theory. It also says it's a technology preview though, so I wouldn't know what to expect for stability.
As a side note, java itself is always 32 bit (if it's spec compliant). The jvm program itself may run with 64 bits but the java program will still be 32. That probably doesn't have any impact on what you're doing though...