How does a video card work?

It depends on the specific model. DELL contracts several different manufacturers for their laptops and each has different habits of removable or not.

- AFAIK most of the XPS series have graphics cards soldered to the motherboard, but there are a few different motherboard revisions, so you can upgrade the motherboard E. G. m1530 has geforce 8400m or 8600m versions. m1730 has removable card and can be upgraded to 9800m GT or GTX SLI but only a DELL m1730 card. M1710 is actually the same as a DELL m90 or something and uses a few removable cards.

the studio XPS 1640 has motherboards with ATI 3670, 4670 and 5730.

-DELL Alienware m15x, m17x and m18x use MXM 3.0 B graphics just like most Clevo/Sager and MSI gaming systems. You can find and change out high end cards like AMD 6990m and Nvidia 580m in all of these.

- DELL Precision m6500 and m6600 also use MXM 3.0 B cards, come with professional Quadro and Firepro cards but can be changed for others. The m4600 uses smaller type MXM 3.0 A.

Anything released earlier than these laptops cannot use different graphics cards or can only use cards made for the specific model.