Any way to get Directed I/O (aka vt-d / SRIOV) on PE2950?

I'm running ESXi on a PE2950 with dual Xeon 5150s and when I try to work with DirectPath I/O it tells me that 'host does not support passthrough configuration.'

Is this a limit on the PE2950 itself due to its own chipset?  Or is it a CPU limitation?

I suppose my question really boils down to: has anyone gotten this feature to work on a 2950?  And, if so: what CPUs do you have?

Thanks!