Memory, if you're using 2 video card for you dual-monitor, BOTH your good video performance will drop to slowest video card performance. Check this link,

http://www.realtimesoft.com/multimon/guide/

Check their software call UltraMon who realy good. If you need more info, drop me a PM. I'm been using dual-monitor with a Matrox G550 for 2 years now so I could help you more.