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-CPU-/250806345830
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.