The recommendations I've read say you need 1GB RAM for system use (assuming a dedicated file server), and then as much RAM (ideally ECC RAM) as you want to give it for caching data.
If you're short on RAM (sub 4GB), you might need to change some of the default settings to avoid problems, but RAM's fairly cheap nowadays, so unless you've got an old machine, it's not likely to be a problem :)
I'd point out that cheap ram is about $10/GB right now, and a cheap HDD is $30/TB. So if you need an extra GB of ram for each TB of storage, you're increasing storage costs by a third.
If you're short on RAM (sub 4GB), you might need to change some of the default settings to avoid problems, but RAM's fairly cheap nowadays, so unless you've got an old machine, it's not likely to be a problem :)