You won't use a Skulltrail board or FB-DIMMS then. It'll be a Xeon 5500 series, with a dual LGA-1366 board. The RAM will be in multiples of 3 as well, so 12 or 24GB are the logical choices.
Besides, Nehalem Xeons are the way to go for that kind of build (the 5500 series for LGA 1366). They're a LOT faster than the older Xeons they replaced.