The Sega Neptune was a Genesis and 32X combined into one console. Sega had admitted that the 32X was rather expensive and had problems, so combining it with a Genesis and selling it at a better price was a good idea. But by the time they had made a prototype, the 32X was nearing the end of its short life-span and the Sega Saturn was almost due for release, so the idea was scrapped as Sega believed there would not be much interest in it. If it had made it to store shelves, the Neptune would have cost around US400.
Sega Neptune Technical Specifications
Sega Mega Drive/Genesis Technical Specifications

" CPU: 16-bit Motorola 68000 running at 7.61 MHz
" Co-processor: Zilog Z80 running at 4 MHz (Not Present in model MK-1631) controls PSG (Programmable Sound Generator) & FM Chips
" RAM: 64 Kbytes
" ROM: 1 Mbytes (8-Mbit)
" VRAM: 64 Kbytes (Video Ram)
" Graphics: VDP (Video Display Processor) dedicated video display processor for playfield and sprite control, 3 Planes, 2 scrolling playfields, 1 sprite plane
" Colours Available: 512
" Max. Colours on screen: 64
" CRAM: 64 x 9-kbit (Colour RAM)
" Pixel Resolution: 320 x 224, 40 x 28 text display mode
" Sound: Texas Instruments PSG (Programmable Sound Generator) TI 76489 chip, Yamaha YM 2612 FM chip
" Signal/Noise Ratio: 14dB
" Sound Channels: 6 stereo sound channels
" Sound RAM: 8 Kbytes
Sega 32X Technical Specifications CPUs: Twin Hitachi SH2 32-bit RISC processors running at 23 MHz, 40 MIPS
Co-Processor: 32X VDP (overlay Mega Drive/Genesis video)
Video processor: 50,000 texture-mapped polygons/sec, texture mapping, hardware scaling and rotation
Colour Palette: 32,768 simultaneous colours on screen
Resolution: 320 x 224 pixels
Memory: 512k (4 MBit) additional RAM
Sound: Stereo PCM chip (audio mixing with Mega Drive/Genesis)
Sound Channels: additional 2 channels
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