The performance hit is negligible, if one exists at all, while using dual screens. I’m currently running a setup similar to the one you’re proposing. LCD as primary and CRT as secondary and using a spanning desktop.
As far as an LCD goes, I can wholeheartedly recommend the Samsung 193P.
A couple review links: (Note: I think these were reviews of early test units)
[http://www.anandtech.com/showdoc.html?i=2024](http://www.anandtech.com/showdoc.html?i=2024) (Anandtech actually recommends this LCD in their weekly buyer guides quite often now as well.)
[http://graphics.tomshardware.com/display/20040326/lcd-04.html](http://graphics.tomshardware.com/display/20040326/lcd-04.html)
It’s very bright, the colours are true if you take the time to properly calibrate it, and if it’s important to you at all, the response time is good enough to play anything without any problems whatsoever. It is just one sexy screen
You may notice some very faint ghosting when you first get it but that passes quickly. (ie. Though I still use my CRT to play Quake 3, the LCD can handle it quite well even when it’s set to contrasting colours like this: http://synapse.redphive.org/q3/q3-2.jpg). Playing games with actual textures minimizes any tiny ghosting effect that you may encounter and manages to hide it 
The problem with a lot of the specs monitor manufacturers throw out at people is that there’s no real “set” way to rate things. One screen could claim a response time of 16ms, but look worse than one that’s rated at 20ms. Usually companies will list the White/Black to Black pixel response time, but the real important one is the Grey to Grey, as that’s the one that takes longer and is usually the cause of any ghosting you’ll see.
I was in the same boat for awhile. I’m an animator and I game. I game a LOT. I also program as well now, but any text looks magnificent with Cleartype enabled on an LCD so that’s not that important. I was desperately searching for a monitor that I could say “yes, this is exactly what I want.” But unfortunately I was always torn between all the supposed specs, reviews, dead pixels, warranties, and real user experiences. Quite often if you were looking for something that gave you true colour representation, you’d have to sacrifice response time, and vice versa. Basically my decision came down to the Viewsonic VP201B, Dell 2001FP, and the Samsung 193P. I chose the Samsung and have had no regrets. This screen has reached an excellent balance of both and I have no hesitations in recommending it to anyone in the market for such a monitor.
P.S. It came with zero dead pixels. I have one pixel stuck on red on the far right, and one in the top right corner. I can only see these if I switch to a white background and then search for them for awhile. (It doesn’t take that long to search since I know where they are now :))
I don’t mean to sound like I’m trying to push this monitor on you, It’s just the only LCD I’ve constantly used can really give personal feedback for.
Btw, this is an excellent little app for any dual monitor users: http://www.realtimesoft.com/ultramon/