Revamp current Network

Currently I have an old network set up in a house that I just isn't cutting it and would like some advice on replacing the components.

The Current Steup:
We have a DSL connection running into an old D-Link 24 port switch which runs wired Ethernet connections into 19 rooms. We also have a Linksys WRT54GL hooked up to the switch, flashed with DD-WRT firmware, and equipped with 3rd-party extender antennas. All of this is centrally located on the top (third) floor, the building is about 140 ft by 80 ft in area. At any given time there may be 10 to 40 different users on the network, most of which is through the wireless.

The Problems:
The wireless connection has poor range and cannot reach the ends of the house. The 3rd floor is really the only floor necessary to get wireless but it is divided into about 18 rooms so there are a lot of walls to go through.
The connection drops frequently making long term use impossible.
The wireless speed fluctuates from unbearable to usable.

What I'm looking to Know:
What are the problems with this network set up?
Do we have too many users for a typical DSL connection to handle?
Do we have too many users for the wireless router to handle?
What would be the best solution for under $500?
What would be the best solution if cost wasn't a factor?
How can I provide reliable wireless to the entire 3rd floor?

Thanks so much.