It's just a souped up Dreamcast with a Cartridge slot. The like Kyuusaku said, the Dreamcast dev box works just fine.
I'm sure there are additional pieces of hardware and no doubt a ton of necessary documentation and Atomis specific libraries but I've sure never seen any of that.
Atomiswave is really really similar to DC See those spec http://www.coinopexpress.com/products/pcbs/atomiswave_system/Atomiswave_Mother_Board_3027.html you can buy one