Several questions here, so I'll do my best to address them all.
First, SmarterStats does in fact run under 64 bit and it is actually our recommendation to do so. 32-bit processes in Microsoft.Net are limited to 1 GB of ram TOTAL and can not exceed it, even for a moment. 64 bit removes that limit, so the process could get temporary memory while it's trying to import logs with lots of complicated data.
OutOfMemoryExceptions can happen because of legitimate out-of-memory errors, but also because of file corruption in your original zip files or in your site data files. Microsoft.Net is interesting, in that they use OutOfMemory for some cases where it's not really an out of memory problem.
In your case, however, it does look like an out of memory problem. You can confirm by looking at process monitor while it's running. I doubt that it's due to your original logs being zipped (that adds very little overhead to the import, memory wise), but rather you just hit a critical mass of data.
Aside from changing your server setup, make sure that you keep up to date with stats. In recent builds we've made some pretty hefty enhancements to its memory usage.
If these tips don't help, you can always start a support ticket with our staff. That will enable them to contact you about getting a copy of your SmarterLogs so they can run it through some simulations here to see what's making it take so much memory.
Hope this helps.
Grady Werner
SmarterTools Inc.
www.smartertools.com