If u intend to game on dual monitors,then buy a pair of R9 290/290x or a pair of gtx780's. A single 290x or 780Ti can handle games on dual monitor.
But if u want to get max. performance,then get a pair of 780's or more powerful 780Ti's. But i would recommend doing SLI retaher than xfire bcoz. amd cards suffer from microstuttering when paired. I didn't think that they slved their problem with the new R200 series gpus.
Two 780's or 780Ti's wouldn't be a prob. as u have a huge budget.
To ur qs."I've heard there's some problems with sli because of overheating. I just want the best performance without over clocking as possible. "
there is no such problem, it can arise due to insufficient cooling, so get some good liquid cooling and good PSU as u have the budget.