Routers actually have multiport switches. Each port will auto adapt to the speed of the device it's attached to. That means a PC with a GigE on port 2 will get 1 Gbps and a PC with a 100 Mbps NIC on port 5 will get 100Mbps. The 100Mbps port won't slow everyone else down.
One thing to remember, going Gigabit ethernet won't do a thing for your internet access. It only affects devices on the ethernet switch ports. You'd have to be running a *ton* of traffic on a PC to see any differences with a Gigabit ethernet NIC.
The WRT54 is an old platform and upgrading to a newer device would buy you a better and faster CPU in the router and that *would* help your internet performance. Older routers with slower CPU's have higher latency than newer units. Getting one with Gigabit interfaces may buy you lower the latency and a faster internet. But you may not notice anything PC to PC.