No reason at all. You'll just need to use the drivers for the PowerEdge 2950, as Dell does not officially support anything other than 2003SS on the NF500 (the NF500 is IDENTICAL to the 2950 other than the name - marketed specifically for the OEM-only Storage Server).
Assuming you have a PERC 5/6, you can use drives up to 2TB in size. If you already have 6TB, you already probably know that it does not support UEFI, so you cannot boot to a "disk" (VD or array) larger than 2TB. What is your planned/hoped upgrade?