<p>What are differences between enabling and disabling "Enable 32-Bit applications" in websites application pools under IIS7 on x64 machine?</p>

<p>Mostly I'm thinking about memory usage.</p>

<p>Thanks in advance</p>

I believe the current "trend" for hosting like this is to have each site with its own Application Pool, and each application pool running in 32 bitness. This enables Application Pool isolation, and allows each site/application pool to be limited to 4gb of memory, which basically works out to about 2.4 GB before asp.net recycles given the 60% memory threshold.
If you have an individual site that requires more memory, you'd change that application pool to x64, allowing for a much larger application pool.