I know it can be done. If I had the spare money lying around for an extra graphics card and monitor, I would. However, I also know that it isn't the easiest thing in the world to set up. Xfree86 has lots of good features, but easy, straightforward configuration is *not* one of them.
I have done it for some time, it has gotten easier with latest distributions. (Mandrake, in my case) The howto littleman pointed to looked helpful, be prepared to have to edit the XF86config file. I like the Matrox dual head videocard solution, but have also done it with two video cards, can be much more trouble.
I did this on my test Linux box here with Mandrake 9.0 and the newest Linux nvidia Drivers. I'm pretty sure the nvidia help files that come with drivers explain what you have to change in the xfree86 file. It was only one or two lines to change to get it to work. I'm not sure what vid card you have but if i'm sure it would be the same for ATI cards, etc... as well.