Why does my system lag so bad when I use two monitors?

[Nitro]

I found XP's Horizontal Span mode with one graphics card and two monitors to be far superior to Vistas Dual Monitor mode with 2 graphics cards and 2 monitors.

Is there any way to avoid having system pop-ups appear in the middle of the setup so the dialogue boxes get cut in half? Because I find that highly annoying. Running vista atm and tbh I'm a bit tired of the poor performance.

I've not tried it but my wife uses UltraMon and I believe I saw settings for soemthing like that.