have you checked out the ASUS Hyper M.2 Card? It might be what you're looking for to split the PCIe slot into two x8 lanes. There are also some helpful YouTube videos from Asus on how bifurcation works. Good luck with your build!
this was suggested in another comment as well. Will check if the bifurcation works only on the hyper card or if other slot can be put in x8 mode.
For Intel 1700 I only know of the ASRock Z790 Taichi (the Z690 model also).
I expect it'll only be found on the most expensive boards unfortunately and note the downsides, lose some M2 slots - although it has loads.
You may not need 8x anyway if you use a PCIe 3.0 NIC, 3.0x4 is 32GT/s (almost 4GB/s) in each direction, plenty for dual 10gbps.
I'm sort of in the same situation, but not sure if the dual 8x could be useful for 25Gbps etc in the future on the other hand the Z790 is 2-3x more expensive that the more basic boards with 16+4 and 2 or 3 M2 slots.
I'm not sure how common bifurcation is on Intel boards, but it could be an option, I worry about risers/mounting the cards though without blocking other slots.
Bingo!!
I will check the board manual for confirmation.
Reg why I am looking for x8 slot is that all the dual 10gbps cards I found are pcie3 x8. Can I use an x8 card in x4 electronic slot and will it have half max bandwidth ie 4GB/s instead of 8GB/s? or system doesn't recognize it?
PCIe is forwards and backwards compatible, so you can use a 3.0 card in a 2.0 slot or vice versa but if the board doesn’t have physically 8x or 16x slots it can require opening the end of the slot.
A lot of 10gbps NICs are 3.0 8x, but I think that is likely so that they still operated at full speed even in 2.0 slots which were probably more common ~10 years ago when such cards were new.
Apart from boards with two x8 slots it could be an option to go with a cheaper 16+4 board, HBA + NIC. With the HBA in the future you could add a SAS expander for more drives plus there are the onboard ones - some expanders can be powered via the PSU and don't need a PCIe slot.
For the sake of $100-150 I would probably go with a board with two 8x ports, it gives more flexibility in the future such as the option of a second HBA instead of an expander.
Looking into the ASRock Z790 Taichi Lite and it's predecessor a little more feature/spec wise it appears they are identical and the difference is cosmetic, losing some RGB and heatsink mass/covers.
Seems like I am missing something
Yep, consumers CPU have very limited PCI lanes 16-24, the main slots (common usage graphic card ) is x16, then the cpu only have 8x lines for the rest of the stuff. Sometimes NVME or extra sata port usa some of that lines
No matter how many slots the MB have, they going to have 4 or 6 lines for the rest of the slots. Normally when there is a extra x16 is x4 only.
The best choice with consumer grade cpu/mb probably is buy a MB that support bifurcation in the main/16x slots. Probably not going to be cheap.
Then probably you going to have problems if you want connect in the future a 10gb card too, if you are planning that check used Xeon/MB normally have plenty of pci lanes (check intel ark)