Rarely but it depends on the particular laptop. On most, it's soldered to the main board so there is no way to upgrade it. If your laptop has a daughter board for the video chip and they offer other graphics options, it's possible.

Pretty much no. With the prices these days you might be better off getting a cheap desktop solution then upgrading it with better parts. If you want to do self-upgrades, i would avoid Dell desktops though.

It is just about unhead of. The whole concept of a laptop is to shrink everything down as small as possible and put it on very few boards. Just adding a connector to allow something like a video card to be swappable takes up a lot of real estate in a tiny world. Laptops are also generally very difficult to disassemble. Laptops are not targeted to consumers who plan to modify them. If you find one that has a swappable video card, I would be amazed. It simply does not fit with the laptop design goal.

If your laptop has an express port, you could always look into an external graphics card.