Technically, he'll do fine running on 2GB if that's what he chooses to do. But the question is what does he plan on doing with his PC? Then you ask, how much RAM is enough?
We know 1GB is out of the question so please, DO NOT get 1GB or you're dead in the water. You will DIE!!!! (ok, I'm being extreme here, but for performance sake, please get more).
What about more than 1GB? Ah, now we're in the sweet spot. Doubling it will notice a real change. I honestly think that the average user won't notice too much of a difference in the speed and stability of 64-bit Windows Vista with greater than 2 GB of RAM under normal load of most applications and gaming these days.
However, if you're running like Virtual Server or VMWare and a virtual machine or two, that's where you can really shine with 4GB+. Having 4GB does give you a lot of headroom to play with for the future since you can throw just about anything at it. Keep in mind that 64 bit apps are here and many more to come in the future.
On a technical side, for 64-bit computing, more memory isn't just a perk, it's a little more of a necessity at the same time. You see, each instruction code, memory address, etc. will take up more room in your RAM and cache due to the increased length of pointers, protected space, etc. Whereas a 32bit OS may be able to execute a program in 100MB, you can find that the size of a 64 bit OS running the same program can take 105-110MB. So more RAM is good.