The most important thing is to install PHP accelerator like APC. If you're concerned about speed, it's crazy not to use one.

PHPTAL caches compiled templates on disk. Using memcached for template cache would be slower. Opcode accelerators like APC have their own in-memory cache and — most important — they store PHP code as compiled opcodes.

PHP has no way of storing compiled code in memcached, so the only way to use memcached is to eval() read templates, and that is the slowest way to run PHP code. Just letting APC to cache on-disk files is the fastest option.