Currently, 16GB Ram on my system. Using up 10.69GB of ram right now running some options expiration effects models. (Mac Pro tower) 5.3GB free.
3gig max is usually what can be addressed through a 32-bit OS, so having more than that is not that helpful.
64-bit dual Xeon box has (came with) 24GB RAM. All other 64-bit Intel single CPU boxes have 8GB of RAM and all 32 bit boxes have 4GB.