You can refer below link on details for replacing CPU on T420

https://www.dell.com/support/manuals/en-us/poweredge-t420/t420ownersmanual/processors?guid=guid-37619bb5-f143-4f06-b7bb-ec2b0509b496&lang=en-us 

Changing the processor will not impact the operating system installed. It will continue to work

Oh even better, I'll look into those processors then! The manual only lists the 8 core versions, but this is good news too!

E5-2450L V2 and E5-2470 V2 processors are also supported on T420 which are 10 core per processor

fyi, before upgrading the CPU, make sure the idrac and bios are at the latest. Sometimes older versions of bios dont have the microcode to support later CPUs that came out after the platform started shipping. Trust me, its a pain to upgrade the CPU try to power on the system and the bios doesnt recognize the new CPU, then you have to remove the new CPU, install the old CPU and flash the bios and idrac, and then install the new CPU. 

Good point, I will check the BIOS and iDrac versions to make sure they are both up to snuff before i go about it

I just checked the iDrac version and it looks like i have the highest that i found on Dell's site which is 2.65.65.65 and for the BIOS, I have the latest of 2.9.0. So i should be all right there. I'm just happy the 10 core 2470 processor is compatible, especially since the manual goes up to the 8 core 2450.

I forgot to mention, if the idrac fw is more than 4 versions behind, its best to step them up to the current level. It takes longer, but is safer than big jumps. Idrac updates dont require a system reboot. Technically, you can just upgrade the bios to make the new CPU work. 

@ the.mknabster. FWIW, I upgraded my T320 along exactly the same lines as you - obviously staying with the one CPU.

So, standard E5-2407v2 to E5-2450v2. (I'd have liked the E5-2470v2 for the 10 cores, but was "tight", and figured for what I wanted to spend, the 2450 was enough of an upgrade (8/16, versus 4/4).

As stated earlier in the thread, "upgrading" is as simple as powering the thing down, removing power cords etc. removing old CPU, installing the "new" one/heatsink, and powering it back up.

My BIOS is at 2.9.0, but my iDRAC is at 2.30.30.30 (anything later has the fans spinning at max. because I have non-Dell HGST drives in the machine. Didn't affect the upgrade either way. YMMV).

Oh so you just upgraded the single processor in your case? I was planning on going dual CPU with either the 2450 or 2470. But you brought up a good point, I didn't think about if adding a 2nd CPU would alter how quiet the machine is, because that's why I have it. I actually have the same non-Dell HGST SAS drives in mine as well. 

The T320 is effectively a T420 with one socket "removed" (and the corresponding 6 memory slots). So, a single-socket box, with 192Gb RAM limit. I doubt that the 2nd CPU will be an issue for noise as such. Many (most?) people only seem to run into noise issues when an IDRAC version complains about non-Dell drives, and spins the case fans to max. (and leaves them like that ) I've tried a number of iDRAC versions, but anything beyond 2.30.30.30 spins the fans to max. So, I've left it at that. Perhaps Dell will eventually introduce a version which doesn't exhibit this behaviour? (Perhaps they already have?)