What is the Max Memory on an Inspiron 6400?

The official answer from Dell documentation and from memory supplier Crucial is that the Inspiron 6400 can use a maximum of 2 GB.  If you search long enough you may find someone who claims to have 4 GB installed, not impossible since the Mobile 945PM and GM chipsets are specified for a total of 4 GB.  However, chipset capability is not the only thing that determines the capability of the mainboard in the computer.  If you decide you want to try adding more than 2 GB, make certain your vendor has a liberal return policy in the event you can't get 4 GB to work.

Second comment on this is that you should be running the 32 bit version of Windows 7 on this machine.  Even if your particular processor is capable of handling the 64 bit version, the 4 GB limit on the chipset prevents you from having more than about 3.25 GB of memory available regardless of your choice of operating systems.  Therefore there is no reason to use a 64 bit OS on the Inspiron 6400.

2 GB of ram is the max that you can use. Since you are running Win 7, do as I have done, plug in a 4GB flash drive and dedicate it to 'Ready Boost'. This gives you another 3.75 GB of useable Ram. It makes a noticeable difference.