Hello Clamped_IT, You can put in a SAS 6gb card into the 2950 & get the PV 124T LTO5 to run as that is a valid setup.  What you are going to need to do is to make sure that you have a free PCIe 8X slot available as the SAS6gb card has to go into the 8x PCIe.  Now depending on your risers that are in your 2950 they come in 2 different options see below: Three expansion slots on two different riser options: Two 64-bit/133 MHz PCI-X and 1 x8 PCI Express full-height, half-length or 2 x8 PCI Express and one x4 PCI Express Here is a link to our compatibility matrix & if you look on page 8 it shows you which card that is needed for an PV 124T. ftp://ftp.dell.com/Manuals/Common/powervault-124t_Reference%20Guide_en-us.pdf Please let us know if you have any other questions.
We have a Poweredge 2950 currently running W2K3 SP2. The server already has a PERC 5/i Controller and a Adaptec SCSI 39320A Ultra320 SCSI card. Is there a SAS card I could purchase for this server and give the capability to connect to a Powervault 124T with a LTO-5 SAS drive? 
Poweredge 2950 & SAS card compatability