I'm looking at getting a second monitor to go with my 21" LCD. I read somewhere (Wikipedia?) that a second monitor hurts the overall system performance because the GPU is pushing data out to two monitors. Is this true? Is it a serious hit in performance/framerates?

I won't be gaming on both monitors, probably one playing games and the other just surfing the net, chatting, whatever.

Unless something has changed I believe that if you run a game in full screen (rather than windowed) mode, anytime you try to interact with applications on the second screen the game will minimize.

I could be wrong or there might be a way around it, but this used to be the case. Personally thats enough of a deal killer for me....

Yeah, based on my understanding, you'll pretty much have to run your games in windowed mode in order to be able to use the other monitor, otherwise you'd have to alt-tab out.

Also, you're right, it would hurt performance somewhat. Imagine a video card driving two 1600x1200 desktops instead of just one, it's obvious that it's displaying more information and "working harder". However, for normal use it'll be fine, and it'll also be fine for gaming depending on how high-end your card is and how high you want your game settings.

i use 2 22" samsung monitors with a 9600GT and i play games from time to time

i must say
there is a very noticible performace hit while playing games

well first i must say you cannot play a game in full screen mode and do something else on another screen
your mouse wont even scroll there
you have to alt tab out first for your mouse to be able to get to that screen
but what i do is i use it to display my msn, itunes, or benchmarking programs on it so i can see real time which song i am playing, who just called me on msn, and how is my computer performing.

unless you run your game in windowed mode
you can control other applications with out having the system load in and out of full screen mode (can be a pain sometimes) to control other app.s but you still have to alt tab out

the second thing you will notice is, when you run your game at a different resolution or color scheme than the second monitor, the nvidia driver screws up the second monitor so much. Dunno why that is an how

and the third thing is if you have SLi or Crossfire, i dun think you will even be able to run dual monitors hahha

anyways
dual monitors is very efficient when you are trying to work
but if you just want dual monitors to play games it would not be a very good idea

I run two monitors off my c2d/8800gts320 setup. I did the same with my old Nvidia 6800/AMD AthlonXP.
I'm inclinded to disagree with the above posters. Here's my evidence:
In the case of both of the above setups, I configured my computer and games using one monitor and only later added the second monitor. When configuring my games, I would first OC my equipment, then test the games that I played until I found an acceptable configuration. I am very consistent about the FPS I want (and I use FRAPs during the configuration phase, so I know what I'm seeing). Anything around 40 is sufficient, so, except with older games, if I'm getting 60+ FPS even in intense scenes I will turn the AA settings up until I don't.
If there was a significant performance hit to adding a second monitor, I would assume that my settings would suddenly be too demanding, once I plugged the second monitor in. However, I have not experienced that. I *did* have to turn the settings down when I recently bought my 1920/1200 24" LCD - but no more than I would expect, considering the resolution change from my 19" LCD.
Note that I am running in Single Display Performance mode, as numerous reports have suggested that Nvidia cards in Multi-Display Performance mode suffer greatly in cards.

I will test this later today by removing my second display from the setup and comparing either FPS in a game I am very familiar with or a 3dmark score. I'll post back then.

I use 2 19" Viewsonics; one for games, the other for temp monitoring. I don't really see that there would be a big hit unless you tried to play 2 games; one on each monitor. (I have tried that...works OK, but not a good idea.) The only game I know of to date that uses 2 monitors is Supreme Commander and does help a lot in that game. Otherwise, a large single monitor does well (my 37" LCD and 42" plasma do well)

i use three monitors for gaming but i have an extra devices that makes windows think it's just one huge monitor... i don't play many modern games (Unreal Tournament 2004, Unreal Tournament 3, Age of Mythology, Age of Empires III, and Maple story in 3 virtual machines at once)...

but i definitely love having the extra information while I'm gaming =]

I tested my 2 monitor (19" + 24" ) setup with 3dmark.
In the both tests my 8800gts was configured for "Single-Display Performance".

In the first test, both monitors were on and the 3dmark06 test was run on the 24" LCD.

In the second test, the second monitor was disabled and turned off and the computer was rebooted.

The scores were exactly the same (over 4 runs, the average was 9620 and all the runs were within 10 points; the highest and lowest scores were both with two monitors connected but the differences are not significant).

The scores were also identical to the scores that I remember getting when I first purchased the equipment, when I only had one monitor, so I dont think there is any chance of some residual setting coming into play and preventing a FPS boost when I disconnect my second monitor and reboot.

In other words, there is no consequence to running two monitors, according my tests, provided, of course, you are only playing the game on one monitor.

i have a gtx 260 running my main mointer that is a 22inch lcd that i use for gaming and a GF 7800 in the second pcie slot on my 790i running my second mointer a 19 inch crt that i use just for system temps and computer info like load on cpu/gpu/psu/ram while i'm gaming
this way all my grapics power of my GTX 260 are used Pushing games on my lcd at there max. only thing is cpu is running a Game and temp and info programs but hay my quad core can handle it.