yung gusto mong sabay gamitin hindi pwede yun. what you can do is if your video card has more than one connector (i.e. merong VGA port, and meron din HDMI/DVI port) dun mo kabit both monitors mo, pag kinabit moa ng isa sa onboard and ang isa sa discreet, isa lang mag magkakaron ng display (usually it will be the discreet one). as mentioned pag merong discreet video card, the default setting is the onboard video card will be disabled
as for the gamer and SLI comment, you dont really need that kahit gamer ka. the performance boost isn't that much of a big deal. gawin mo lang yan if puro high end ang components ng computer mo AND you have cash to spend. you will only enjoy SLI if ALL your components are really high end so you can put max settings on games. otherwise, one card is more than capable of playing the game at high settings. if you have an existing card and thinking of going SLI by adding one more of the same card, i would highly advice you check a single new card first. the gains of going SLI with your existing card might not be worth it, baka mas ok pa pag isang bagong card na lang na higher sa current mo
dun naman sa 3 monitor output, this depends on your video card, the newer cards (nvidia) support up to 3 outputs already so no need to SLI (kaso sobrang high end na yun, you might be better off going SLI). kung AMD, then they have eyefinity