Make sure you get a quality power supply. A bad PSU is hard to debug.
Google 'microstutter'. I would not xfire 2x $75 HD 5750. Instead consider an HD6850 also at $150
"used mainly for gaming" the balance of graphics vs CPU is off. A $900 system should have $200-$300 in video. A $700 i3 based system with a $300 video card will outgame your $900 system pretty badly.