I don't know jack sh*t about motherboards. On the compatibility page for MBox for Windows XP, it says that you need to have ATX form factor motherboard. But according to the folks at Dell such a motherboard only exists in desktop computers. Since I want to get a laptop, am I correct in assuming that whatever motherboard form factor comes in the Inspiron XPS is the same as the Inspiron 8500 (which is reccomended on the MBox compatibility page, but no longer sold by Dell)?

you should never assume when it comes to issues as important as your motherboard. Call Dell and ask them.

atx is a standard form factor - like a light bulb or a roll of toilet paper. every roll of toilet paper will fit any TP holder and any standard light bulb will fit any standard size socket. every atx motherboard will fit any atx case and accept any atx power supply connector. laptops are different. i suggest you check out the "best laptop for PTLE" thread for best compatability and value.