I haven't tested many games on my K6-3+ but I'll share what I have. Generally with late 90s 3D games I think my super 7 can do 15-30fps with a Geforce2 MX. That's playable but if you're looking at it from a modern 60fps perspective, it might not cut it. Maybe Voodoo3 would do better, I didn't get that far with it.
K6-3+ 450/100, VIA MVP3 chipset, Geforce2 MX, sound disabled:
In UT99, sound disabled: UTBench.dem was getting 20fps at almost any resolution at 450MHz.
Driver, sound disabled: 17fps at 450MHz, 18-19fps at 550MHz. However it should be better in realistic gameplay and at lower settings. This was on high settings in a specific, maximally demanding scene that I was able to reproduce with lots of cars piled up around me. It was probably a worst case situation.
Ultima 9, sound disabled: 450MHz mostly 9-11fps in a specific scene near the start of the game. It was even slower with some driver versions, that score was with the more optimal ones. But that's a notoriously slow running RPG that nobody cares about. Voodoo3 is probably faster, haven't tried it.
Gabriel Knight 3, sound disabled: Tested a specific scene, not sure how representative it is. 35-65fps depending on resolution and video driver version. 65fps was at 640x480 with NVidia driver 8.05.
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I'd avoid the original K6-3, instead look for a K6-2+ or K6-3+. You have to make sure the board can run them though (Vcore and BIOS support). Lots of boards have unofficial BIOS patches to run them. Lots of boards also have undocumented Vcore settings.
I had a K6-3 since new, but ever since I replaced it with a 3+ I'm a lot happier with that PC. It's a much cooler, more stable, and faster system now (at 100FSB). At 66FSB you won't get any more speed than an original K6-3 can give you (because 6x is the max multiplier), but it's still cooler and more stable anyways, and easier on the motherboard to run it.
My K6-3 was originally a 450MHz 2.2V chip, but it hasn't been stable at those settings since it was 6 months old (if it ever was - back in the 90s we blamed crashes on Windows). Heat was always a hassle with it, and to think most of them were marked for 2.4V, as mine apparently should have been. The K6-3 is the Prescott of socket 7.
Last I was aware, the K6-2+ chips are widely available, but the 3+ are harder to find. Pretty much every "K6-3+" eBay listing is keyword spamming to sell a K6-2+, but there are exceptions where actual K6-3+ chips show up. The K6-2+ are nearly as fast as the 3+, so unless it's going to bother you not to have the very best, then the 2+ is a good option.
Regardless of how AMD rated them, the "+" chips can do around 550MHz at 2.0V or less in a desktop. They're the pinnacle of socket-7 manufacturing.
The original K6-3 are uncommon, expensive, and inferior so they're not worth using IMO unless you really can't run a "+".
Since you already like your socket-7 system, I'd probably go for it because these chips are great for maximizing the range of such a system, especially in DOS. The "+" chips are particularly good because they allow changing the multiplier (and the cache, but not sure if both levels) in software, so you can set up batch files to throttle it down when needed. It's hard to beat these systems for DOS, and if it leaves something in Win9x that you aren't happy with, let that fall to a faster Windows machine.