From a quick peek at wikipedia, i'd guess the regular ram is DDR 2 and the other is DDR1 - it seems to match up with the big SVG (linked below) on the page
Incredibly useful diagram showing pinouts
SVG version here if you need it bigger showing the pin positions for being sure. - its an SVG so i can't link it directly. Also, the notch on the ram is always on the same side - count the number of pins to be sure.
The systems are probably from different generations and just happen to use utterly incompatable forms of standard ram. It would be easy to check from an OS - using LSHW in linux or WMIC or a front end to that in windows