1, I'd say the Intel SE440BX-2 Slot 1 motherboard. 1 x AGP, 4 x PCI and 2 x ISA. Very stable, not the best for overclocking but these days with slot 1 cpu's being so cheap, why would you not choose the speed you're after and forget about overclocking altogether. Also as I recently discovered, this board will take a 1ghz 100fsb pentium 3.
Intel SE440BX-2 + Pentium III 1ghz Slot 1 CPU?
Oh and you can still purchase them brand new in box from eBay.
2,Single cpu. Windows 95/98 cannot use more than one cpu at all. Only an NT based OS can use dual cpu's. NT based systems will also not give you 100% compatibility with all win9x games.
3, If your adamant that you must use a Geforce256 then I'm of the same opinion as retrofanatic. Never owned one before.
4, The sound blaster live 5.1 would be best given your cut off date for this build. You just have to be careful as there are so many different models to lookout for. I believe mine is the SB0060 model. Great card for win9x gaming. Yes the audigy is better but it was made in 2001 I believe.
Now I do have to ask, are you not going to be doing any dos gaming at all? Is there any reason your parts have to be limited to Dec 1st 99? If you don't care about dos at all then as retrofanatic suggested, a socket 478 board would be best. I'd go for the Asus P4P800-SE as I know first hand this board will take a 3.4ghz Pentium 4, has 8x AGP, uses an intel chipset and is windows 98 compatible. You could also go with a Sound Blaster Audigy 2 ZS Platinum and a nvidia 6800 Ultra or ATI X850 Pro. Both of these graphics cards were the last windows 98 compatible graphics cards with drivers still readily available. This option would allow you to play "all" win9x games on maximum settings as well as games made all the way up to around 2003-2004.