First of all, have you tried installing DD-WRT or another third party firmware on that router? The WRT54G is probably one of the most popular routers ever released because of how well supported it is by third party firmwares. I'm personally a big fan of DD-WRT, but Tomato was also designed around that router as well. Depending on which version you have, the WRT54G can be a real beast when set up properly. If you're done with it anyway, you lose nothing by trying out another firmware, and you may save yourself some cash.
Having said that, I know the previous poster said don't skimp, but I say skimp skimp skimp. I recently picked up a refurb
WRT160N for $30 and could not be happier. Installed DD-WRT right out of the box and have been amazed at its performance. And at that price, I'm actually thinking about picking up a few more and using as wireless bridges instead of buying $30 wireless adapters that will perform significantly worse.
You get more features as you move up in price, but I think a lot of those features are lost on most people. Do you really need gigabit network speeds? I personally don't move around a lot of big files, my primary use for the network is media streaming, 10/100 is more than enough for that. Do you really need dual band networks? For reasons I don't understand 5 Ghz has become really sexy in the last couple of years, and while it has some notable advantages, it has some notable disadvantages as well. Are you experiencing a lot of interference at the 2.4 Ghz spectrum? In my case the answer was no, so one of the biggest advantages of 5 Ghz was a dud for me. If you are worried about weighing down a single network with all your advices, just turn your WRT54G into a wireless access point and have two networks running, one g, one n. Do you really need file servers and printers, keeping in mind you can typically set these up yourself much better than the majority of routers out there can via their firmware? For me, again, the answer was no.