The NAOMI (New Arcade Operation Machine Idea) was an arcade architechture released by SEGA in 1998, and as metioned above, was broadly similar to the Dreamcast. However, it had significantly more main and graphic memory. Acording to SEGA, Naomi is Japanese for "beauty above all else", though I've been unable to confirm this.

Main Ram : 32 megs
CPU : SH-4 64-bit RISC CPU (200 MHz 360 MIPS / 1.4 GFLOPS)
Graphic Engine : PowerVR (PVR2DC)
Sound Engine : ARM7 Yamaha AICA 45 MHZ (with internal 32-bit RISC CPU, 64 channel ADPCM)
Main Memory : 32 MByte
Graphic Memory : 16 MByte
Sound Memory : 8 MByte
Media : ROM Board (maximum size of 168 MBytes) / GD-Rom
Simultaneous Number of Colors : Approx. 16,770,000 (24bits)
Polygons : 2.5 Million polys/sec
Rendering Speed : 500 M pixel/sec
Additional Features : Bump Mapping, Fog, Alpha-Bending (transparency), Mip Mapping (polygon-texture auto switch), Tri-Linear Filtering, Anti-Aliasing, Environment Mapping, and Specular Effect.

The Naomi uses ROM boards, up to 168 Mb, or GD-ROM media. There are over 60 games on available for Naomi, but only a few (House of the Dead 2, Crazy Taxi, Capcom vs. SNK) are well known outside of Japan.

The Naomi 2 is an upgraded version of Naomi, with a much more powerful graphics engine (quadrupled polygons per second and rendering speed compared to the original naomi).

CPU : SH-4 128-bit RISC CPU (200 MHz 360 MIPS / 1.4 GFLOPS)
Graphic Engine : 2 x PowerVR 2 (PVR2DC-CLX2) GPU's
Geometry Processor : Custom Videologic T+L chip "Elan" (100mhz) -
Sound Engine : ARM7 Yamaha AICA 45 MHZ (with internal 32-bit RISC CPU, 64 channel ADPCM)
Main Memory : 32 MByte 100Mhz SDRAM
Graphic Memory : 32 MByte
Model Data Memory : 32MByte
Sound Memory : 8 MByte
Media : ROM Board / GD-Rom
Simultaneous Number of Colors : Approx. 16,770,000 (24bits)
Polygons : 10 Million polys/sec with 6 light sources
Rendering Speed :
2000 Mpixels/sec (unrealistic max, assumes overdraw of 10x which nothing uses)
Additional Features : Bump Mapping, Multiple Fog Modes, 8-bit Alpha Blending (256 levels of transparency), Mip Mapping (polygon-texture auto switch), Tri-Linear Filtering, Super Sampling for Full Scene Anti-Aliasing, Environment Mapping, and Specular Effect.

Naomi 2 was released in 2001, and has only a handful of (stunningly beautiful) games, including Virtua Fighter 4, and Virtua Striker 3. I've never seen these here in the UK, and they may be Japan only ATM.