If there was a WRT54G version with Gigabit ethernet, it would be my daily driver today. Having a 100mbit/sec switch is the only reason why my WRT54G is sitting in the shelve without any work. I only use it for tinkering one day or the other.
You can install openwrt on x86. Grab an old desktop PC (or laptop) and if it has built in GigE it might be enough, depending on the processor. Worst case buy an Intel NIC and you're off.
Alternately get an Edgerouter X and install openwrt on that.
Can confirm this router can handle gigabit ISP speeds, I upgraded to this specifically because my previous router (also flashed with OpenWRT) couldn't get my full speeds I was paying for.