All original 800v4 units and those that had the 'full fat' upgrade should already have the VI00 card installed.

If your unit does not have the VI00 card, then good luck in finding one! MrTechGuy may be able to assist as they purchased all Chris Coasters spares and he used to have some old 800-series bits.
If you find one, you just need to plug it in and tell the unit in the configuration program that it has been installed. Then you can set video sources for each source.

You can technically have the 800 work as a pre-amp, as you can install IA00 (Analogue inputs) and IE16 (Digital inputs) cards and even an OA20/OA23/OA24 output card for analogue outputs, although there is no 'surround' processing internally.

Good luck,

Steve

Main: 861v6 (LPS), 808v6, UHD722, DSP8000SE Anniversaries, DSP5200.2HC, DSP7200.2, SW1600, Sky Q, Cambridge Audio BD, Roon, M80
Office: 861v4 (LPS), 818v3, 800v4, HD621, M&K S150P, DSP5500HC, M&K MPS2810 sub x2
Son's system: 861v4, HD621, DSP6000.2, DSP5200HC, DSP33
Spare: MCD, 100 Series, 200 Series, 500 Series, F80s, C15s.