OK, how about "one third of your total memory should be physical"?
Like everyone says, I always configure a generous swap, but it is rarely used.
Code:
Device 1K-blocks Used Avail Capacity
/dev/ad4s1b 3897344 68K 3.7G 0%
4GB amd64 box. That said, it runs squid, and I am more than happy to have it caching in RAM to it's heart's content.