I've never understood this "switch in the BIOS" fascination - it's a royal pain in the neck, and it will not defeat the additional cards under Windows (it just picks whichever card is initialized for POST and that will carry over to some bootable environments, like UBCD). I do not remember how 9x handles different graphics adapters with different drivers - I know 2k/XP generally doesn't care unless they create a conflict (I've never had issues with ATi + nVidia, 3DLabs + 3DFX, etc). You also won't have the pass-through feature that Voodoo/Voodoo2 offers, so you will either need a switch for your monitor (and there's going to be display outputs that will then exist in the ether, or you'll have to disable them in Windows, etc), or multiple displays, etc. Basically this sounds like more hassle than it's worth to me. I also don't think a GeForce FX or above is really necessary for a Pentium III, unless it's all you have. Why not just go with the Voodoo3 (or a 4 or 5) and be done with it? I think if you're primarily targeting Glide games you should be AOK, but if you want DX8.1 and DX9 and all that under XP you probably also should look at a faster CPU (P4 era) and a powerful AGP card (like a GF6600).
Power consumption wise I'm not sure - I've never known my single Voodoo2 12MB to really draw that much power, especially when it isn't rendering, so I can't imagine adding a second one would really kick things up that much (when they're doing 3D it might, but that's probably not 24x7). I know that I had no problems with a P3 1GHz, WildcatVP 880Pro, Voodoo2 12MB, SB Live, and 3 hard-drives on a 200W PSU (and the machine never even drew 200W at the wall - usually it was like 100W draw, and this is an old PSU so it may be 50-60% efficient). Swapping all the graphics cards out for a Voodoo 3/4 would probably lower power draw (and decrease complexity), but going from a DX7-8 AGP card + Voodoo2 to DX7-8 AGP/PCI card + Voodoo3 may end up with higher power consumption at the end of the day. But that's just a guess.