How do I limit internet speed to certain person?

Yes you can (probably). The WRT54G (which I have), has a QoS section under "Gaming and Applications", over on the right side...

With QoS, you can limit certain ports, mac address or application to certain priority.

Setting yourself to high priority should give you the bandwith when you need it, but when you are off, whatever is left over is given to the remaining computers.

While not actually controlling speed, you can control who gets priority for network traffic....

I'm assuming the WRT54G2 is the same as my WRT54G with these settings....

Depends on your router but there may very well be settings there on the advanced page. Look for terms like QoS. That's the only way without software.

Probably not without custom firmware on the router, so I guess the answer is no, since you'd have to "install" that, in a sense.

You wouldn't have to install anything on your computer though.

Yes, if your router is capable of QOS (quality of Service). You can adjust priorities for devices connected to your router.

since you do not want software then only hardware is left

so you need custom hardware which can only use software to limit speeds

No way to specify who gets how much of the "speed".

No.