The third party firmware will work on pre version 5 G models or the GL. Linux was referring to the OS that the router itself is running, later models of the G changed to VxWorks as an OS. You don't have to be using Linux as a PC OS to use the GL.
Yes, 54 is referring to the wireless g standard 54Mbps. Linksys do wireless N (or strictly pre-N as it's still in draft form) routers, details
here. Again, it would be for future proofing, as you'd need a compatible wireless N adaptor for your laptop.
I have a Linksys WRT54G on the VM 20Mbps service & I get 19Mbps through the wireless link. Having said that, I'm about 10 feet away from the router in line of sight!