I have three pairs of 12mb Voodoo II cards: Canopus, Diamond, and Creative.
Back in the day, I paid through the nose and actually purchased a new pair of Canopus Pure3D II cards and ran them in SLI. After upgrading to a Voodoo 3, I sold them on eBay and suffered "seller's regret" shortly afterwards. I then purchased another used pair on eBay. In hindsight, this was a really good idea. Nowadays, even when you can find a Pure3D II card for sale they are quite costly. They also use a proprietary VGA pass-through cable that other Voodoo II cards do not. I haven't used them in awhile and I keep them in a box in my closet. I'm very hesitant to sell them because I know I'll never find another set like them again.
My Diamond cards are in use and work well. I purchased these used on eBay shortly after the Pure3D II cards back in the late 90s. Back when TennMax was still selling their Stealth coolers, I considered purchasing a set. However, I didn't like the high cost or that the fans on them don't seem to be easily replaceable. There are also no passive cooling fins on the other parts of that Stealth cooler. Instead, I attached some heatsinks to the cards using Arctic Silver thermal epoxy. I don't overclock the cards or anything, so I hope that maybe the small heatsinks will extend the usable life of the cards.
My Creative cards are in rough shape. I purchased these on eBay about three years ago when you could still get a Voodoo II card for about $20-$30. One of them arrived smelling like mold and looking like it had survived a flood. The other had nasty scratches all over it and looked like it had spent a lot of time in the bottom of a box with other cards. I was pleasantly surprised when they both proved to work just fine. I also attached some heatsinks to these cards to prolong their lives, but they look real ugly.
Of the three sets, only the Canopus Pure3D II cards came with fans installed on them. The Diamond and Creative cards did not. As far as I know, the Canopus cards were the only ones to include active cooling right out of the box.
I'm sure that Voodoo II cards will work just fine without any fans or heatsinks as long as there is some airflow in your PC case and you aren't overclocking. Still, unless you're keeping them as part of a collection, why not attach some passive heatsinks and keep 'em a bit cooler? It's cheap, easy, and might actually help a bit.
There are so many options on eBay for passive heatsinks. This one looks good, for example. You'll want three of those per card, though. At $3.99 each, that's gonna cost you about $24 to cover both cards.
Here's what I used for my ghetto Creative SLI setup: Small RAM heatsinks, blue or silver, 8 per package for $2. You'll attach four of these per chip, for a total cost of $6. It's real ugly, but it works.
I'd really like to find a pair of 12mb Orchid Righteous 3D II cards, but I don't see them for sale very often. Diamond, Creative, and STB seem to be the ones I see most frequently. All of them are pretty much the same, except the Canopus cards that deviate from the reference design.
My ideal Voodoo II setup would be a pair of Canopus Pure3D II with passive heatsinks and a Spectra 2500 primary VGA adapter. There is no need for an external pass-through cable in this configuration and VGA signal suffers no degradation. However, you would need to permanently deface the voodoo cards to achieve this. For such a rare card, I can't bring myself to do it. The Spectra 2500 is also somewhat hard to obtain. I wouldn't bother with this configuration in anything faster than a 1ghz coppermine. I also wouldn't bother with my "ideal" Voodoo II setup because it's impractical and there are better options for legacy gaming. For any CPU up to 1ghz, I would just use a Voodoo 3. The performance is better, it's a simpler setup, and it only uses a single slot. It is also substantially cheaper to just buy a single Voodoo 3 card. For any faster CPU, I would use a GF3 Ti500 or GF4 Ti4600 as primary vga and SLI Voodoo IIs for GLIDE compatibility.