According to estimates, this is need for SQL Server:
- up to 50 users (or small databases) - minimum 4GB
- 40-80 users (or medium databases) - minimum 8GB
- 80 and more users (or large databases) - minimum 16GB
These are minimums, and the basic rule is that the more is the better. I usually MINIMUM twice multiplie these numbers.
Absolute minimum based on MS System Requirements is 2GB, but it's funny, except for the really small databases.