1950 v1 servers will only support 5100 series CPUs.  1950 v2 servers will support 5100 and 5300 series CPUs. 1950 v3 servers will support 5100, 5300, and 5400 series CPUs. If you want to convert a v1 into a v3, all you have to do is change the motherboard.  1950s are so cheap nowaways, you can probably buy a 1950 v3 with CPUs for about the same pirce as just the motherboard and avoid having to do the upgrade.
The PDF from dell says 5100 series and 5300 series for v1, and v3 supports up to 5400 series. However there's reports of 5300 series cpu's not being supported in v1. If that's the case, can I drop a III series motherboard into a first gen 1950, or are there other physical changes to the system that would prevent that from working? I'd like the 5400 series, but at minimum I need quad cores, which 5100 series doesn't offer if that's the only series supported.
Poweredge 1950 v1 processor support