Well - For anyone else out there as clueless as I.
It *looks* like the 7300s were Tigertons and are older and the benchmarks I finally found put it significantly under the 5500s (Nehalems). The 7400s are the dunningtons which have up to six cores - but strangely don't kill the Nehalems even though they appear to be quite expensive.
I never really found exact benchmarks for 4 cpu boxes - but I'll have to just pull the trigger and hope because I am out of time.
There aren't yet any Nehalem-based CPUs that run in any more than a two-way setup. You have Core 2-based units like the Xeon 7300 and 7400. The first Nehalem that will work in 4-way and better setups is the Xeon 75xx "Beckton."