There are a couple of options, a "dedicated computer" would be good, but I would check your wireless settings, see what speed the laptops are connecting. It really sounds like it is a router issue.
If you decide to go with wireless N try Frys/outpost, they seem to have good prices on their house brand airlink and that has worked (as peripherals, actually I use a netgear router) You will get increased speed even if you do not upgrade the laptops, but better if you do.

Second, you might just try adding more memory to the current main computer.

Third, while a 3G machine will be somewhat faster, any dedicated machine would be better.
I also like pacificgeeks.com http://www.pacificgeek.com/product.asp?ID=94333&C=220&S=982
this machine is $129, but you would need more memory.

Another option would be to install a version of windows on the powerserver and use it as a regular machine peer to peer.

Wendell
Pediatrician in Chicago

The patient's expectation is that you have all the answers, sometimes they just don't like the answer you have for them