hmm for processor upgrade I think on the future a K6-III 500MHz since the motherboard suport it 😁
But I think a pentium mmx 233MHz plus voodoo 3 2000 will do the job (I got surprised with this setting performance, A good hard disk and do a really difference, even with 64mb of ram 😳 )

I would prefer to build a P233 MMX system for later DOS games and for Windows 95 multimedia gaming rather than those 3D intensive games. For those 3D games, I would prefer to build at least a fast PII or even better, a PIII based system.

From your list of cards for that P233MMX though, I would choose the Matrox G450. I'm not sure which Diamond 3D you listed, but if it's Diamond Monster 3D, I would put that in too.

in fact the pmmx has very strong fpu performance. a pmmx overclocked to 300 can calculate superpi in less than 6:50, which is a few seconds faster than k6-2-550, and 1 min faster than k6-3-300!
however when it comes to newer games like 3dmark2000, it will fall behind the k6-3 due to lack of 3d instructions.