Ok, I was in your situation once and let me tell you, it isn't what its hyped up to be, don't get me wrong, if someone were to take my 2nd monitor I'd probably end up crying or murdering them. lol.
But as far as gaming and doing other things (i.e. chatting): NO
However, its very good for schoolwork and desktop applications and things like that, the problem with dual monitors is that each monitor is still acting as the same desktop.
So lets say you're running Counter-Strike in a window on your desktop, if you click out of the window then Counter-Strike will be an inactive window switching to whatever you clicked on, so think if you were playing counter-strike fullscreen, you would click somewhere in your second monitor and it would minimize counter-strike and you'd be in your chat window, etc. Deeming the 2nd monitor (during gaming) soely on temperature monitoring and I use it for viewing my playlists in Winamp.
Now there may be some sort of program out there that eliminates that, but from what I know all I use is UltraMon: http://www.realtimesoft.com/ultramon/
As for the videocard, go with the 6600GT, I am running a 5700LE and a PCI GeForce 2 MX400 and it runs pretty good for a bare minimum 2nd desktop.