Specs

The MX 400 List

0.18 micron manufacturing process
200MHz core clock
166MHz memory clock
2 pixels per clock cycle
4 texels per clock cycle
800 Mtexels/s fill rate
2.7 GB/s memory bandwidth
20 million triangles/sec
8-64MB frame buffer
128-bit Single Data Rate (SDR) or 64-bit Double Data Rate (DDR) memory
TwinView architecture
Digital Vibrance control
Integrated Dual-Link TMDS transmitters
NVIDIA Shading Rasterizer (NSR)
High-Definition Video Processor (HDVP)
AGP 4X with Fast Writes
32-bit color
32-bit Z/stencil buffer
Cube environment mapping
DirectX and S3 texture compression
350MHz RAMDAC

Here is basically a summary of what the differences are between the cards; we're going to toss in the GTS for kicks:

GeForce2 GTS
200 MHz core
32 MB 128-bit DDR SDRAM (166MHz)
Memory bandwidth - 5.3 GB/s

GeForce2 MX400 200MHz core
64 MB 128-bit SDRAM (166MHz)
Memory bandwidth - 2.7 GB/s

xGeForce2 MX (the original)
175MHz core
32 MB 128-bit SDRAM (166MHz)
Memory bandwidth - 2.7 GB/s

GeForce2 MX 200
175MHz core
32 MB 64-bit SDRAM (166 MHz)
Memory bandwidth - 1.3 GB/s