very heavy and alot of traffic are abstracts that no one can answer.
You will need to provide at the very least:
- architecture (64-bit apache+php consumes a lot more memory per process)
- ammount of physical memory
- CPU speed, number of CPUs, number of cores per cpu
- FreeBSD version
- Expected traffic (not 'alot', but requests/second, breakdown static/dynamic content per hour at peak)
To paraphrase what you asked:
"What tires and engine do I need for my car to be able to drive to my mom who lives far far away?"