The two graphics cards you just listed are exactly the same. The MX4000 is the direct replacement for the MX 440, just a cheaper part to manufacture now...
As for performance, I can't see any difference between them either.
I would recommend the FX 5200/5600 instead or the ATi 9200/9600 graphics cards. All of these come in either 64MB or 128MB versions, I recommend the 128MB version.
You'll be glad you spent a little extra for the added 3D hardware support and performance.