How many of those also work with transparency over them and also blend icons against the live video?
The WinAmp milkdrop plugin handles transparency (when it exists) in XP, as tested with the ClearOne Winamp skin and WindowBlinds, and flawlessly with all transparency enabled applications in Vista. My AGP 8x machine was the one I ran XP32 on when I used to do this.
In Vista, for example, I am using Winamp/Milkdrop on the desktop right now on my third monitor (adapter 2) at the same time that DeskScapes is playing Photo Dream on my monitors 1 and 2 (adapter 1). I've tweaked the visualization to run on Pixel Shader 3 at 1680x1050, with all internal settings maximized, playing at 30 fps and reacting to the music playing in WinAmp, of course. Here is a screenshot.
As far as Icons go, XP wouldn't antialias/blend the edges over the milkdrop background.
In Vista the icons flash on monitor 3 when "show desktop icons" is turned on in the visualization plugin, but I'm not sure if that is an issue with my unique setup (which also has Fences running, though I've hidden the icons in the screenshot). They don't affect my monitor 1 & 2 (adapter 1) icons, etc. Windows, gadgets, programs, etc. all behave as expected.
I haven't run XP in a while, but I do remember that VLC desktop wallpaper worked under XP. I'm pretty sure WindowBlinds transparency did work with it as well.
In Vista, VLC will turn off Aero acceleration in Vista to play in the background. However, I have been able to play multiple instances of HD video simultaneously on this machine. Though, if it's all software accelerated (as noted above) that might make sense.
I will try and boot into XP again later today and see if I can give you guys better answers on that front.