I was hoping someone might be able to help me identify a modchip. It belongs to a bloke I know (friend of a friend sort of thing), and all he can tell me is that when he turns on his xbox, he gets the "xecuter rox my box" logo. I know this occurs with both the x3 and 2.6 Is there a way to work out which chip it is without opening it up?

That logo tells you nothing about the chip - it just means you are running an X2.5xxx or X3.xxxx BIOS.

If you have an X3 there wlll be an X3 switch panel somewhere on the Xbox, and an X2.6 will have an X2.6 switch panel. It is possible to fit them on the inside of the Xbox, but usually they'd be on the outside. If you can't find a switch panel then you might have a DuoX2, but the only way to know for sure is to open the Xbox.

This page has links to pictures of the X2.6 and X3 modchips along with their external switch panels.