Memory is cheap these days - you can get 24GB for a couple hundred bucks depending on speed/ECC requirements. I always try to max out the memory based on the median cost per GB.
Those "ECC requirements" really make a difference though. And whoever is running servers without ECC memory in production should be shot dead (just wait until you have data corruption because of a faulty memory module to see what pain is).
Yep - we just bought 54 x 8GB DDR3-1333 Registered DIMMs for about 10K, and we can sell the 54 x 4GB modules we are replacing for about 2-3K on eBay. But since this is a VMware cluster, now I have to scramble to see what this is going to cost me in additional licensing before I upgrade. :(
