For the Sound Card consideration, it's going to depend on specifically what you are after.
The "Diamond Monster Sound MX300" is one of the highest end cards of the time period, so it works great with Windows 95. the MX200, it's predecessor, is a good one as well. Both support Aureal 3D, an early 3-D Audio API, and the MX200 even has a daughtercard you can connect for better MIDI samples. They are effectively rebranded Aureal Vortex 2 Cards, so that could add additional selection while looking for pricing options on eBay.
The main problem is twofold: first, because they are some of the "highest-end" cards for older computer builds, they tend to fetch a tad more than one might expect on eBay. Another issue is that they have poor DOS support. (the first Vortex (not Vortex 2) and MX200 do have Sound Blaster Pro emulation/compatibility under DOS, though)
Sound Blaster AWE64 Gold is a good option For MS-DOS games Sound Blaster support is pretty much a given, and the AWE64 is 100% compatible with the AWE32, which many games have added features for. It does require AWEUTIL.EXE, which is part of the driver set, to get General MIDI support under DOS but that works reasonably well. It also has good Windows Support.
Another excellent card is a Sound Blaster Live! Card. the only issue with that is that it was a very common "OEM" card, that would be pre-installed, and the pre-installed cards are stripped down and aren't actually SB Live cards for some reason. (My Dell Dimension 4400 had one in it and I could only ever get t he driver that was for the Dell Dimension 4400 to work; the default SB drivers for the Live! failed to install, claiming it couldn't find a Sound Blaster Live! installed), so going that route you'll have to avoid OEM cards.
Another card I'd say to avoid would be "Creative Audio PCI". This is a rebranded Ensoniq Audio PCI, and both are incredibly basic cards. They get the job done, so they will do in a pinch and they are cheap, but if you are looking for a more "deluxe" option for the time period, it isn't this card.
For Graphics Cards you have a lot of options. You can find Cards from NVidia, ATI, Matrox, and 3DFX that will work on Windows 95. 3DFX Voodoo Cards are 3-D Accelerator only and need to run with another video card for your standard desktop display, and use a pass-through adapter. You can also install two of them for SLI configurations.
Like Dave Says, 3DFX cards are somewhat collectible now and have a cult following. Personally I think they are somewhat overrated in terms of actually making use of them, unless specifically wanting to use original hardware with a Glide-supporting game. You also need another Graphics Card anyway for the Voodoo and Voodoo 2 cards anyway.
If you want something for all those operating Systems, then something like a Radeon 9000 or Geforce 2MX/Geforce 4Ti should work well. I use an Radeon All-In-Wonder 9000 on my own Pentium 4 PC.