This is from the manuals.

Processor Upgrades – Power 2950 II
andPowerEdge 2950 III Systems
•If the front of your system chassis is labeled with a “II”, your system is
upgradeable to the 5100 series of dual-core Intel Xeon processors and
the5300 series of quad-core Xeon processors.
•If the front of your system chassis is labeled with a “III”, your system is
upgradeable to the 5100 and 5200 series of dual-core Intel Xeon processors
and 5300 and 5400 series of quad-core Intel Xeon processors.
See support.dell.com for information on the latest processor upgrade options
for your system.

Well given that it came with a Quad Core CPU I will make the assumption that it is a III, but the two new CPUs I've installed it doesn't seem to like.   Could it because the X CPUs are a higher voltage to the E CPUs? 

Well given that it came with a Quad Core CPU I will make the assumption that it is a III, but the two new CPUs I've installed it doesn't seem to like.   Could it because the X CPUs are a higher voltage to the E CPUs?

Actually, if you currently have a Xeon 5300-series quad core, your server may be a "II" and not a "III".

If you currently have a Xeon 5400-series quad core, then you can safely assume it's a "III".