setting up a wireless file/backup server

I don't see any reason to buy new wireless cards for your old machines. File serving and internet access and anything else you do over the network can all go through the same wireless (or wired) "pipe" simultaneously. The only problem might be a speed problem-- The older wireless cards might not give you the data transfer speed you would like. If you want to upgrade to new wireless cards for speed reasons, make sure your wireless hub is using the same or faster wireless protocol. BTW why aren't you just using a wired network connection to the old machines?

 

Yeah, as far as bandwith issues go you're better off using a modem with wireless output for your internet signals and connecting your servers to that (preferably via ethernet, but wireless will be ok). You can access them through the network, but your internet signal/band will be a lot stronger.