For multiple x16 pcie you'll need threadripper or equivalent.

Real world for gaming in even the latest gpus x16 isn't needed, x8 is fine.

I think the nvidia 690 and 790 chipset that support SLI it does have 32 lane of pcie which can provide two full size x16 slot for SLI support.
https://www.anandtech.com/show/2482/2

If you want to play with SLI then the nforce board should be the safe choice although they tend to dying due to chipset heat.

for intel, there are x38/x48 chipset that have 32 lane pcie which also provide 2 x16 slot with full bandwidth but they mostly support crossfire.
for amd, according to wiki, they have crossfire express 3200 with dual x16 support, the 790FX, 890FX, 990FX with dual x16 slot or quad x16 slot with x8 bandwidth

If you really want to go back in time and grab a period-correct board, the Asus A8N32-SLI Deluxe will do that for you. That's a socket 939 nForce 4 SLI x16 board.

Also, any reputable X79 board will do that; SB-E and IVB-E CPUs provide 40 PCIe lanes.

X58 chipset boards on lga1366 also usually have 2 pcie x16 slots. And they support sli and crossfire.

As for running in pcie x8 mode, the performance difference is typically negligible. That's if using the same generation slot and gpu (ex: pcie 1.0 gpu on pcie 1.0 slot).

A 7800GTX is barely gonna notice running on 4x vs 16.

There were some tests recently where 3060 was just barely slower on 8x. So yeah, nice quest and all, but it's a bit like looking for an ATA133 interface for your 386.