Check ebay and you may find some good deals. I have the Netgear PE102 and am very happy with it. Other options include the Linksys hpb200 and hpro200. The latter can also function as a router.
For cheap, your best bet might be a 2wire homeportal 100. It can function as your router if you want it too. Newer versions have an option to configure it in "pass through bridge mode" or something like that, which basically means that you can kill the router functions and use it only as a bridge. With older versions, though, you can't do this (and with any version the way to do this is not well documented so write back if you get one and need details). The 2wire is meant to be a router for idiots (many dsl companies sell it) and can be maddening for more advanced users, but basically the trick is just to ignore its cutesy software and do everything yourself. Because of the bios issues, I would aim for brand new rather than used.
I agree that getting a bridge is a good idea, regardless of your OS. No need to mess with drivers or software -- your computers won't even know that hpna is involved, to them it will all just be ethernet.