Hello Martman5,

Inspiron 6400 was shipped tested and shipped with 2 kinds of processors. 

1. Intel Core Duo Processor T2300 (2M Cache, 1.66 GHz, 667 MHz FSB).
2. Intel Core 2 Duo Processor T7250 (2M Cache, 2.00 GHz, 800 MHz FSB).

Hence I would like to say that upgrading the processor to Intel Core 2 Duo Processor T7250 (2M Cache, 2.00 GHz, 800 MHz FSB) would defiantly work fine as it has been tested on the system. 

Hope this helps!

The Intel T7200, T7400 and T7600 are the three fastest ones you can use.

If I install a T7600 do you think it will work? Do you think my bios will recognize it? can it handle 4 gigs of ram. Thanks for your help.

Hi,

Thank you for the response. I think you're right the specs look similar except for the speed increase.  The T2300 can only handle a max of 2 gigs ram. It looks like the T7600 can handle 4gigs. If I install this chip with 4 gigs and it works, then the 6400 would have some life left in it.

The system can take up to 2G of RAM, max, regardless of the CPU - at least officially.  4G may or may not work - but they chipset is old enough that you'd see only about 3.2G of the 4, even with a 64-bit OS.

Yes, the T7600 will work in this model. It's the fastest of the socket M CPU.  Just BE SURE you get a genuine T7600 - not a remark or an engineering sample (which won't have a clock lock and won't work).