It sounds to me like the only way you may keep this client is to explain that with only one of the win98SE limitations (ie 2GB mem, nForce 4, SLI, or dual cpu) there could be hacks that MAY make the system useable. Combining all of these multiplies the odds by a magnitude that the resulting system will never be unusable as a dependable system and cannot be supported by a professional such as yourself. No one supports this; not MS, not Nvidia, not AMD, and not you.
You will lose the cust. either way if he insists on this. If you try to make it work, it sounds like you have some suffering in store for you even if you get it to stumble along; and I bet he'll tell everyone! If you refuse, even if he tells everyone, they'll think, " Of course he wouldn't do that."
Your only good option (and best service to the customer!) is to convince him this is wrong. Remember this is why people come to you; to keep them from building an incompatable system. Build a lessor win98se machine if one is required, but this one requires XP SP1 or higher.
Jim