konradh
I have 12GBs of RAM because I was told I needed more RAM to run a lot of virtual instruments; however, I have never seen my RAM usage hit even 6. (By the way, "a lot of virtual instruments" means 6 to 12 instances of soft synths, like RealGuitar, Ivory II, two or three Kontakt instruments, EZDrummer, Vienna, etc..)
It's instruments that load samples into RAM and stream from there that need lots of RAM. If you're streaming the samples from disk, then the only RAM that gets used is to store the attacks. This lets notes play immediately when you hit a key, while the hard drive seeks the samples.
I believe Kontakt lets you disable DFD so you can stream from RAM instead. This will make life easier for your hard drive and probably allow for more audio tracks. Also note that some hosts do not stream from disk (Reason used to be that way, I think it still is) and in that case, having lots of RAM really helps.