If so, can they support unlimted number of overlay planes? What bit depths?
I am developing under Mobility Radeon 9600, Radeon 9700, & Nvidia 6800.
No implementation supports an “unlimited” number of overlay planes. 
None of the above boards offers overlay planes, you’d need a workstation graphics board to get one.
If you want to draw something above a main plane which should remain intact, have a look at the WGL_ARB_buffer_region extension.
It allows to quickly save and restore the color and depth buffers and should work on all of the above boards.
Gamers cards usually don’t support overlay planes in OpenGL. I was do some overlay tests on Quadro cards and it works. Also, using some old driver set (I think detonator 2x.xx series) and RivaTuner nvstrap patch I succesufully turn GeForce3 to Quadro card and voila… overlays work! But… with newer driver releases I was not able to get overlays when I try to turn gamer card to Quadro.
yooyo
You can also use RivaTuner to patch the nv4_mini.dll to use new drivers while modifying Geforces to Quadros