You're only as fast as your weakest link. In this case, your modem. Assuming you had good signal strength on your WRT54g, you have 54Mbps of available bandwidth. Your modem depending on what company and package you have can be anywhere from 1Mbps-20Mpbs or higher depending on your package. Quite simply the WRT54g was more than capable of transferring that amount of data from your modem to your PC.
Why buy an expensive router?

?? Aside from features that people want, like port forwarding, better network management, more range, usb hard drive and printer hosting etc...... Most people need the Quality of Service(QOS) engine. This engine prioritizes who and what programs get more bandwidth. So if your surfing the internet on your computer, and your sister watching netflix on hers, it'll give your sister more bandwidth so her movie will be smooth and stutter free. Surfing the internet can have a little lag. Same for prioritizing gameplay giving it the most bandwidth it wants, and giving whatever's left to bit torrent downloads.
A more expensive router also gives you better intranetwork speeds(traffic between computers in your home). This can be especially useful when doing PC backups on the network, or just sharing large files between computers.
Also with more users connected, and more data going through the router. A router with more processing power and ram can be helpful in maintaing network speed. But if you only have 2-3 computers in the house, and only using the router for the internet. You could have installed DD-WRT firmware( for a good QOS engine) on your WRT54g and you wouldn't see any gains for buying a more expensive router.