The is one of the last of the socket M systems  -- it will take a T7200, T7400 or T7600, but nothing else - no T7x00 with x = odd or above 6, no Pxxxx, no Core 2 Quad, no i3/5/7.  Those three CPUs are 64-bit capable but long out of production - you may still be able to find one on the used market (expect the T7600, which is a terminal upgrade for this and many other systems, to be very expensive, even on the used market).

Thank you for answer.

But what about this system (it's the second Dell I have) (CPU-Z specs follows)

I see the CPU have quite similar specs with the prev one I posted (although smaller speed), and MB uses 945GM chipset instead of 945PM.

Can I use also 7200/7400/7600 in that too? Also, can I "reuse" my T2400 to upgrade a little the smaller laptop?

Thank you.

PS. where can I find more info, eventually comparative, between CPU's Core Duo T2xxx and Core 2 Duo T7xxx? Thanks again

One is Inspiron 9400, other is 6400.

So does they support T7600?

Thanks

I found the CPU on ebay from 230$

And I already upgraded the notebook a little : 4 GB ram and a 160 GB hdd 7200. So I hope to extend its life for 2-3 years with a CPU.

And my plan is to move my CPU - T2400 into 6400, making that one a little faster.

Anyway, thanks for suggestion

Many Core Duo systems can take a COre 2 Duo - some won't though.  What model is it?

You'll have to ask the seller - and verify their reputation.  The price is suspiciously low.

Thank you again for all your suggestions

Thanks for warning me. Any idea how can I make sure it is true T7600?

Make sure you're getting a true T7600 -- not an engineering sample (ES) or a T7600G  (unlocked CPU) -- neither of those will work, and both are very common on EBay.