The Nforce3 is the last Nvidia chipset to support Win98.
BIOSTAR NF325-A7 754 NVIDIA nForce3 250 ATX AMD Motherboard - Retail $40
http://www.newegg.com/Product/Product.aspx?Item=N82E16813138247
Examples of drivers here (WIN9XGART):
http://www.biostar.com.tw/app/en-us/mb/driver.php?S_ID=201
The last S754 processor left on Newegg.
http://www.newegg.com/Product/Product.aspx?Item=N82E16819104249
Board uses DDR RAM, which you shouldn't have a problem shopping for.
S754 is single channel, so no need to match memory sticks (as there
is no dual channel function).
This is not the latest technology, but it might meet the Win98 requirement.
On the Intel side, I'm afraid you may have missed the boat. There were
some 865G boards with LGA775, which would run an FSB1066 processor.
I think they're all but gone from retail. (Asrock had some, Asus had
P5PE-VM, and those would be ones I'd go looking for. I expect a dual
core processor would be "half-wasted" on Win98, but the machine
would scream
)
There might be some older VIA chipsets, maybe K8T800 ? That had AGP I think.
It is a fun research topic, that is for sure.
Note that, it is possible that a lot of motherboards can be
coaxed to load Win98, but the video might be limited to 640x480
the same as the BIOS screens. I suspect that the Intel chipsets
have a compatibility mode, so that disks still work. But video
is the "weakest link". I don't think, if a board has PCI Express,
that Win98 is a candidate. So you're looking for AGP systems, as
being more likely to do the job.
If the system is dual booting, there is a hack for Win98 (MaxPhysPage),
where you can trim down the detected RAM. That would allow a bunch
more RAM to be installed, and then when Win98 boots, it is
forced to only "see" 512MB or whatever you select. Then, the second
OS can work with more available RAM, without having to worry about
Win98 compatible amounts of memory.
Good luck,
Paul