What is your WRT54G connecting to now? Laptops, iPads, etc. Give us a list. If laptops, what wireless radio is being used in the laptop? Sometimes Windows Device Manager will tell you.
WNDR3700 can help in some cases, especially if client equipment is newer. I have had cases where I was able to use a single WNDR3700 in an office to cover the same area that the previous Linksys/Netgear took two units to cover.
But I've also seen it happen where a user comes into the office with a 6+ year old laptop with a "b" or early "g" radio and they wonder why they cannot connect. Half the time its because their client won't work with AES encryption and other times they are forced to sit closer to the WNDR3700 due to their laptop radio not being sensitive enough to receive distant signals. So, check your gear first.
If you go for the WNDR3700 be sure to mount it vertical, away from mirrors and preferably not in an enclosed space. You may need to rotate it to achieve best signal level throughout the house. BTW, I recently installed a single WNDR3700 in a two story 5K sq ft house and it covered the house surprisingly well. Only a couple fringe spots with really low signal.
You can also look at the WNDR4000 which recently released. It uses a Broadcom radio (instead of Atheros in 3700). A couple users on this forum and Netgear's forum claim slightly better range and throughput, but who knows. We need Tim to take one for a whirl.