The more the better, games are always getting better and need more and more.
One of my desktop has 12gigs ddr3, however I don't game on it. It is really used for vm.
I do game on my laptop though and have 6gigs. It is alright however it is time for an upgrade. Ive been looking into a laptop which is custom built, for around $5,000 I can get a 24gig ddr3.

If your into vm there is no such thing as over kill.