Serious Sam runs "alright" on my Coppermine 800 + Geforce 4 Ti at 800x600. In the realm of 30FPS. I don't remember being blown away by high settings and a butter frame rate on SS until I was on a 9800 Pro + Pentium 4 2.8 Ghz. If you can pull off finding a Pentium 3 1000mhz, I think you would find the game runs quite nicely (but still not perfectly smooth).
In those days 20 FPS was considered acceptable for a minimum frame rate. Most of my game programming related books from the mid 90s to early 00s used to suggest aiming for 12-13 FPS as acceptable before things become a slide show. In fact I think my copy of SS TFE may have a PII 300Mhz listed as the minimum. "Recommended system specs" generally was to get near 30 FPS (where today people expect 60).
Amazing how spoiled by frame rates we've become with 144hz monitors and such.
Found a low quality video but you can see the the same video card on a better CPU here.
Sup. I like computers. Are you a computer?