X86 for 1Gb with cake

I would like to get an x86 OpenWrt instance going, that will support 1Gb with cake when I eventually upgrade.

What's the minimum I need in terms of specs?

If we optimised the hot path, properly configured the hardware/software interrupts so on and so forth just about any x86 would pull off gigabit speeds.

Have a look at cpubenchmark.net. As a rule of thumb anything in the ballpark of 2000+ points should be able to bitbang at gigabit.

How about an Atom n2600? Would it support 100 Mbps and SQM?

Well kernel 5.4 helps a lot in that matter. My guess is that a cpu with half that point or less should now be able to cake 1gbit on a recent snapshot.

Yes.
If the system struggles try adjusting interrupts per queue per interface and offloading some stuff to the nic.

I am running an OpenWRT x86 19.07.4 Guest on ESXi.
Hardware is a Pentium G4560 with 2400 MHz dual channel ram and dual Intel nics. 250 Mbit bandwidth layer_cake shaped consumes ~22% total cpu. Software flow offloading is enabled.

So Gigabit could be doable using that hardware when you don't have the virtualization overhead. Just to give you some ballpark numbers.