Performance hit if running dual screen?

If you have 2 monitors and run a game on one with the desktop + apps on the other how much will this affect the performance of the game? Do you need a mega pc? Should both monitors be same resolution?

My speces - e8400 3.0ghz, 4gb ram, GeForce 7950GT 512mb, Win7 x64

Well my friend runs a 17in monitor along side his 22in one and when hes playing games theres no slow down.

Compared to running a game on a single monitor with the apps still open in the background, there is no performance difference.

As long as the card has dual RAMDACs, there's no hit. Most cards do.

Little to no hit.

I tried to run left 4 dead and the desktop on the other monitor, but hte other monitor just went black and showed no input, yet other games worked. :S

so you have two different monitors showing two different things from the same PC? cool and indeed can be helpfulGazaAli

You can even mirror your main monitor onto the other one so they seem the same picture. Good for if lots of people are watching a movie or playing a game around one monitor.

I run dual 22" screens for extra productivity when doing many things. Photoshop in one, dreamweaver in the other. Or productivity tool, gamespot forum on the other :P. Performance can take a hit, but in my case its no big deal. I have my GTX 260 as my primary, and an 8600GT running the secondary (also my physX card). Cheap way to keep the performance up and get a little extra in gaming (some games anyway) :)