It is nothing to do with Windows98 and everything to do with the nVidia drivers.
The graphics card will work with any operating system providing it has drivers for that os to give it the correct instructions.
Same thing goes for SLI.......It's nVidias drivers that give support,not Windows
So if a driver release from nVidia states that it is compatible with PCI-e and SLI and is written for 98/Me then it will work.
Your friend is talking rubbish....as your web search illustrates.
What advantage running an older game in SLI will be is another matter.A single 6800 will run any old game maxed out with ease.
It's also possible to run just about any game from the Win98/me period under Windows XP......I have loads of them.Some work without any tweaking,some need compatibility mode switching on and some (like the original TombRaider games) need patches applying.