Well ya of course you can always look back and say I should have gotten Nintendo stock in Spring of 06 before the Wii was annouced at E3 and the PSP was appearing to be taken a good chunk of the handheld market from nintendo and there was talk of Nintendo going third party "when" the Revolution went the way of the dreamcast... hindsight is 20/20
But for the future, now is the time... if you don't get it now you will miss the boat completely.
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut