Which NICs would you recommend for me to buy for testing at least 1x100 Gbps (ideally 200 Gbps?) networking between this machine (PCIe 4.0) and an Intel Xeon one that I have with PCIe 3.0. Don't want to spend much money, so the cards don't need to be too enterprisey, just fast.

And - do such cards even allow direct "cross" connection without a switch in between?

All 100G is enterprisy.

For a cheap solution, I'd get a pair of used Mellanox ConnectX4 or Chelsio T6, and a QSFP28 direct attach copper cable.

If you care about price, check out (used, ofc) infiniband cards.

They all seem to offer/suggest daisy-chain connectivity at least for those with two ports per card as one potential topology.

+1 on what the sibling comment said.

As for directly connecting them absolutely, works great. Id recommend a cheap DAC off fs.com to connect them in that case.