This is a two part question.
how much memory can I install in this computer, not how much is recommended. Can you go beyond 8G
How much faster is the PC 6400 to PC 5300 chip?
8GB is the max according to HP's website.
You probably will not notice a huge difference in the real world. Here is a comparison from Wikipedia:
The Windows Experience Index for my RAM dropped from 5.9 to 5.3 (If I remember correctly) when I installed a slower PC 5200 module. That's a difference of 10%. It can depend on the chipset, because AMD has a better memory controller. But the price difference is only 1€ for a module, hence it's better to buy the faster memory if you want to be sure, as well as minimize cost versus speed.