The 64MB MX400 does not hold a substantial performance increase over the 32MB MX200. The memory pipeline is simply to small.
A better option entirely is the Radeon LE. I was a bit scared of these at first because they only have one rendering pipeline (compared with 4 on a GeForce 2 GTS), but they seem to perform well, and are really cheap.
I have a TNT2 M64 (second monitor) in a PCI slot next to my GeForce 2 Pro, and sometimes I try playing 3D games on the thing. It stinks (I can't get over 800x600x32 in any games I play without jittering).
~ I know the human being and fish can coexist peacefully ~