Ebay or similar used is the only way I know to get a card these days. I don't have a recommendation for a "general" card, they went through some iterations back then before things settled, and different games supported different cards/card sets.
Best thing you can do is make a top 5 or 10 list of the games you want to play, then find out what 3D graphics cards they support. Hopefully that will give you an idea which the best card to buy (first) is for you. It's been a while, but I've had good luck with my Voodoo3 and a later ATI card and early GeForces - for dos and windows, obviously. My Voodoo 1 & 2 became too much hassle many years ago. However, none of those cards were a "best of" later buy, but an at the time "what I could afford".
And if you're talking dosbox and emulation, it doesn't matter in the slightest. Emulators can't use the card directly.

I think MAME emulation has the best of that kind of support, and they are still all software AFAIK.
Also, has anyone came up with a way to use a second core of a two core processor as a virtual video card?
No. What's your goal, here? Maybe someone solved it a different way, and asking about method is getting in your way. Problem is video card architecture is different than standard general processor - it's hard enough to separate out one core of a CPU, it'd be a bigger task to do that with the specialized GPU chip, and you'd still need full driver support if the hardware supported it. Sometimes you hear about using the GPU for other tasks, like calculating bitcoin hashes or other math (GPUs are good at iterative math!), but that's just using shader and programmable pipeline technology, not actually getting into the processor and doing whatever you want. (AFAIK from what I've read, IANA(GPU architect or programmer)!)
It's not about 3D cards, but there's also this Vogons.org topic about "best DOS video card":
http://www.vogons.org/viewtopic.php?t=14296