In terms of overall gaming performance, the graphical capabilities of the Nvidia GeForce 6600 GT 256MB are massively better than the Nvidia GeForce 4 MX 440.

The 6600 GT was released over a year more recently than the 4 MX, and so the 6600 GT is likely to have better driver support, meaning it will be more optimized for running the latest games when compared to the 4 MX.

Both GPUs exhibit very poor performance, so rather than upgrading from one to the other you should consider looking at more powerful GPUs. Neither of these will be able to run the latest games in any playable way.

The 6600 GT has 128 MB more video memory than the 4 MX, so is likely to be slightly better at displaying game textures at higher resolutions. This is supported by the fact that the 6600 GT also has superior memory performance overall.

The 6600 GT has 12.8 GB/sec greater memory bandwidth than the 4 MX, which means that the memory performance of the 6600 GT is slightly better than the 4 MX.

The GeForce 6600 GT 256MB has 11 Shader Processing Units and the GeForce 4 MX 440 has 2. However, the actual shader performance of the 6600 GT is 6 and the actual shader performance of the 4 MX is 1. The 6600 GT having 5 better shader performance and an altogether better performance when taking into account other relevant data means that the 6600 GT delivers a marginally smoother and more efficient experience when processing graphical data than the 4 MX.