Nvidia 9400M vs Intel HD Graphics 4000 I have done some searching on Google and found some tests saying HD 4000 is a great improvement over the previous generation, but I didn't get any tests that compared it with discrete chips.

Could any of you guys that are more into hardware tell me if the Intel HD 4000 will be better than Nvidia 9400M? 

The integrated graphics is in the Mid-2012 13" MacBook Pro with 2.9 Ghz Core i7, while the Nvidia part is in the 2009 13" MacBook Pro 2.53 Ghz Core 2 Duo.

P.S. I only play Diablo 3 on my laptop. I don't care about graphics as long as the framerate is good. Currently, with the Nvidia chip, it's really bad. It is listed as low-performance on Blizzard's site, but the Intel HD 4000 is, too.
According to the benchmark comparison tool here, the HD 4000 has between 2x and 6x the benchmark scores of the 9400M. (How relevant the scores are to your gaming experience is yet to be seen. Some of these are synthetic benchmarks, so they may not be wholly relevant.) The list is ranked with faster cards first, and the HD 4000 is #181, while the two 9400Ms are at #302 and #310.

tl;dr: It seems that the Intel HD 4000 should be better than the Nvidia GeForce 9400M.

Note: to be able to select the 9400M in the list, you need to deselect the "Still available (not archived)" box to show old cards.