It uses i940GML/i943GML which means you need a Socket-M processor.
I am not sure what other limitations Intel has placed on it when compared to full-blooded 945GM besides 533FSB max.
You should be able to upgrade to Core Duo like T2050 for example without problem. But something like Core2Duo T5500 (SL9SH) might not work.
You need to update BIOS to the latest first before attempting any upgrade.