Intel T4100 processor

The X5DIJ does not show up on the ASUS web site, so I can't definitively check for CPU compatibility. The laptop has a GM45 northbridge and the Celeron T3500 is a 45 nm Socket P processor with the very last stepping of the line (R0), so any of the Penryn-based Core 2 Duos should work.

This laptop seems to have a removable cpu, found a mainboard for that laptop thru ebay.

http://cgi.ebay.co.uk/Asus-X5DIJ-Motherboard-Complete-C...

Now we have to see what upgrade options are.
It also seems that there are various types of X5DIJ's, is there also something behind this number (like SX033C) ?
I am not sure if this secondary number has something to do with the capability of the MB itself but a X5DIJ-SX313V houses a T4400

Further looking learned that the exact type of mainboard in that laptop is ASUS K50IJ and has a LGA775 socket.

You make have a problem with heat if you upgrade.

http://www.ebuyer.com/product/222845
(UK  )
I assume this will work as they are both socket P.

The Celeron T3500 is a Socket P mobile chip, not LGA775. Socket P looks nothing like LGA775.

Sorry must have misread that somewhere then.