Hmm... Well, using a second vid out port on the vidcard is no good if you intend to have each monitor showing something different. If you're looking to mirror the displays (IE what one shows, the other one shows), then you are more than likely going to have to invest in a booster. The problem with this is that the parts are pretty expensive.
This is, of course, unless you are willing to invest in some truly heavy duty cable, as I believe with the proper shielding you can run your video signal upwards of 50 feet. Beware that the last time I ordered this same type and length of cable for a customer, the cost was somewhere in the neighborhood of $280!!
The only other option is to use a booster box, which is even more expensive, and may not be available for DVI connections, or convert the signal to NTFS and transmit it via wireless (ala Radio Shack's video transmitter/receiver units).