Atomiswave motherboards are fairly common. They come up on Ebay all the time. They work fine in standard res, but look much better on high res. An ebay seller, Yaton6262, I believe often sells the I/O cart and gun pcb. There are also often Sports Shooting USA kits that come up from time to time on Ebay. As far as what is a Sega Naomi, that's probably best answered by paying a visit to the Sega-Naomi.com forum. Lots of good info there about the Naomi hardware and games.
A Naomi is basically a Dreamcast and is a pretty fine arcade system if you ask me. If you're in any way a fan of the Dreamcast, then you would probably love to have a Naomi. For more info, check http://sega-naomi.com/
The Atomiswave is based on the Naomi hardware, but has a much smaller library of games. I eventually want to get one, but it's not high up on my list at the moment.
Yes, that is a complete Naomi GD Rom system. Like Mattroid said, the Naomi is basically the same as the dreamcast, even the game library. Most Naomi games were ported to the dreamcast. The GD Rom system usually goes for around 400-500 bucks.