Jan 2001 with Quake 3
2XP3-700E@933 133Mhz bus X 7 1/4
Honestly though didn't enable SMP much in Q3, FPS were higher in at 800x600 and below but didn't game at low resolutions if I didn't have to.
I did see a benefit with everything on the system since it had dual processors so anything else running on the system didn't interfere while gaming even though the games were limited to a single processor which you could change the affinity for which was nice.
Played around with UT on a dual PPro 200 sometime in 2000 but AFAIK Unreal 1 didn't support SMP.
Next processor that had multiple core was in 5-18-2007 C2D E6600, even then barely any games used multiple cores. I seem to remember Supreme Commander having support. UE3 games? Farcry 1, Serious Sam 2.
You probably need to look at 2008+ for a larger list. Mabye track when the game engines started adding support, included Vista on the game box, XBOX 360 ports, etc.