Post 2.4 Linux and Solaris both now generaly recommend 2 x RAM size for swap.
HOWEVER, http://www.cyberciti.biz/tips/linux-swap-space.html says:

Here is my rule for normal server (Web / Mail etc):

   1. Swap space == Equal RAM size (if RAM < 2GB)
   2. Swap space == 2GB size (if RAM > 2GB)

My friend who is a true Oracle GURU recommends something as follows for heavy duty Oracle server with fast storage such as RAID 10:

   1. Swap space == Equal RAM size (if RAM < 8GB)
   2. Swap space == 0.50 times the size of RAM (if RAM > 8GB)

SOLARIS
Per http://blogs.oracle.com/jimlaurent/entry/solaris_faq_myths_and_facts
Rule of thumb 30%.
ZFS not currently support for swap files.  ?


!!!  Post 2.4.10  (per http://doc.opensuse.org/documentation/html/openSUSE/opensuse-reference/cha.advdisk.html )
    The memory management system of kernels before 2.4.10 needed swap as a safety measure. Then, if you did not have twice the size of your RAM in swap, the performance of the system suffered. These limitations no longer exist.


Linux adding swap file:
  dd if=/dev/zero of=SWAP.bin bs=1G count=X
  chmod 600 SWAP.bin
  mkswap -f SWAP.bin
  Add line to /etc/fstab:  /path/to/SWAP.bin swap swap defaults 0 0
    (Copy, paste, edit existing swap line)
  Test fstab:  swapon -a; swapon -s