My vote is for a sub-100 MHz Pentium or 486. I had a 486DX2 66MHz and a Pentium 120 MHz back in the day, but the 120 wasn't out until 1996. Some people forget that after the Pentium (and before the Slot 1 Pentium IIs) came the Pentium MMX, which were Socket 7 CPUs. These were 166/200/233 MHz, if I recall correctly. There was also the Pentium Pro, which was their high-end chip, like the Xeon today. These used a larger socket, called Socket 8. I'd say it's pretty unlikely these are what he had, since they weren't common until 96.
Things have come a long way since then.. video memory was often 1MB or less, and hard drives were still measured in MB, sometimes even if it was over 1 GB (the "1080 MB" hard drive was quite common). There were also the Cyrix CPUs, which is long forgotten by most... they used names like "5x86" and "6x86", which would trick people into thinking they were better than Intel's 486 and the Pentium (which people thought of as a 586). Cyrix was pretty much garbage, though -- the FPU could not compete with the Pentium, overheating was common, and I am pretty sure I can recall issues with using SoundBlaster 16 cards as well. (Onboard audio didn't exist back then)
As for AMD, back then they made a line of processors called the K5, which along with Cyrix they were considered bottom-of-the-barrel compared to an Intel Pentium. These were all Socket 7 CPUs, which of course had the pins on the CPU itself, rather than the motherboard. (It might be surprising to some of you, but in the eyes of many people AMD was considered sub-standard, or just outright garbage in these days... until the Athlon.)
Oh, and yes, overclocking did exist back in these days
Even the old GPUs, like the Riva 128 and Riva TNT (some of nVidia's first cards) were overclockable. I remember popping in a Diamond Viper V330 (NVidia Riva 128) and OCing it for the first time... I thought it was the coolest thing ever lol. Setting the clock rate/multiplier for processors was done by setting jumpers on the motherboard, so overclocking was possible in this way, too. My first "big success" with overclocking was running my Pentium II 400 @ 448 MHz, a whopping 12% gain! Cases back in these days were almost universally "tombs", most wouldn't even have a single fan in there stock.
Anyway, I guess I'll have to end the history lesson here lol. Have fun with those old CPUs.