Giga Sampler was the start of mainstream disc streaming, and it allowed large samples to be played without requiring large amounts of expensive Ram, (Up until recently it was only server computers that could read and use a large amount of Ram) and it basically works like this, when you select a sound, the first part of the sound (Which could be less then a second) for each key is loaded into Ram, (Tip: make sure all the sounds you require for the tune are preloaded into Ram by the VST) when you press the key the first part of the sound is played, which gives the program time to load the rest of the sound off the hard disk, (Streaming) if however you do not have enough Ram, then the program cannot load in enough of the sound to cover the time it takes to access and stream the rest of the sound from the hard drive. (Hence the apparent latency, although it is actually delay)
Most of these programs (Which include Kontakt 2) recommend a minimum of 1GB of free Ram to work correctly; however these types of programs do not normally require large amounts of CPU power.
Emulations and effects VSTs (B4.FM7, Guitar Rig etc) rely on the CPU power to produce and process the sounds, therefore the more CPU power the better. (Large amounts of Ram are not normally required unless a lot of these programs are loaded in at the same time)
As you can see from the above, it is not possible to say how much Ram CPU and Hard Disk is required, without knowing what programs you intend to use, however a good way is to look at the specifications that the VST manufactures say are required for there instruments, then add together the specifications which will give you an idea of how much Ram, CPU and hard disk is required. (You can use the minimum specs as it is unlikely that every note pressed will require the use of every VST)
Don’t forget the OS, and any other samples you have loaded, will also take up Ram and CPU power.
Hope this helps.

Bill