When it comes to PCI SATA cards, I think they will always be SATA-1, and (for the moment) if you stick with a 2-port SATA card for a win-98 system then it will work fine with the drivers that are publicly available. (my 4-port PCI sata controller does work under win-98, but only in PIO mode - not DMA/UDMA mode - I'm still working on fixing that).
SATA-2 expansion cards only come (I believe) as PCIe cards (not sure which type - x1 or x4). Perhaps Rlowe can elaborate on the win-98 driver situation for those types of buses. I would think that the vast majority of single-OS win-98 computers operated by msfn members do not have the shorter PCIe slots (assuming they have any PCIe slots in the first place) and hence the usefulness of trying to find a solution to SATA-2 functionality for win-98 is limited to a very small audience.
I would also think that any motherboard that has the shorter PCIe slots (x1 and x4 slot types) would also have on-board SATA-2 controller. Perhaps Rlowe is saying that he has gotten those on-board SATA-2 controllers to work under win-98. If so - have they been verified to operate in DMA / UDMA mode under win-98?
SATA-3 support under win-98 is even more a non-issue. SATA-3 probably has to be integrated as part of the motherboard (are there any SATA-3 PCIe cards?). Anyone with a SATA-3 motherboard trying to run windows 98 on it probably has bigger issues to overcome - like basic chipset driver support.