Connecting a GPU by Ethernet is like connecting your PS3 controller to the PS/2 keyboard port: sounds like it's trying to fix a problem but the solution turns out to be completely outlandish and impractical. (Granted, if you can pull it off, you'll gain more popularity as that one crazy guy.)
That said, I advise you not to try to do this because the interface simply doesn't exist. You'd have to go through a lot of hoops to get your card to accept low-level commands through high-level interfaces and then bring back a video output. The latency would be absolutely horrible, unless you are willing to pay thousands and thousands of dollars for an enterprise-grade solution that lets you do this for one reason or another.
However, don't fret. You may still be able to connect your GPU to your laptop. For instance, if you have an ExpressCard slot, you can use an ExpressCard to PCIe adapter (along with a power supply unit) to have a seamless experience.
However, there may be compatibility issues due to the operating system's inability to set up resources for the PCI connection to the GPU, so browse around and make sure it's compatible before you buy anything. Consult guides such as PCWorld's eGPU tutorial to see if you have everything you need to use a desktop GPU with your laptop.
Regardless, do not expect 100% performance. Remember, it's a laptop.