Powering up analog part of the video card
I'd go with:


  Use cascaded DC-DC (12->6) and precision LDO (6->5). In this architecture device will have additional noise from the DC-DC converter, and I am not sure if it is better than just using +5 V from the system.


Sure, a switch-mode power supply adds a new source of noise – but that's a source that you can control! Typically, what you'd try is to put that noise spectrally where it doesn't hurt that much (e.g. in an audio application, that'd be above 100 kHz), but in a video application, that might be hard – SMPSes work somewhere between 20 kHz and 4 MHz, and your signal of interest sadly usually spans both.
So, you'd often mitigate the noise by using a slow switching speed, or even spread-spectrum supply, which doesn't reduce the noise power, but smears (spreads) it over a larger spectral range. You'd then passively filter as well as feasible (i.e. here's where solid LC filtering comes in handy), and:

Follow that with a good linear regulator. Don't be satisfied with a LM7805 or alike. Use an LDO whose datasheet actually states noise rejection over frequencies, or shows the output spectrum for a given white input noise. Good LDO output buffering always helps – don't skip on the output cap (typically, use a tantalum + ceramic with each >= 1 µF each), and on the decoupling caps on every single supply pin on your analog part.