DELL PRECISION T5500 - Best configuration in 2020 ?

I'm running a workstation which is used for gaming from time to time. IMHO dual CPU would seem pointless as most games are going to be played on consumer machines so won't be coded for dual CPU as certain workstation applications might be. Even if you do some Maya, Blender on the side it is probably faster to use a discrete GPU than CPU anyway.

What I would first consider before you go down any path is to look at the power supply unit you have in your T5500. For gaming you will need a decent discrete GPU and invariably those will require extra power via combos of  6 or 8 pin connectors. For any of today's games, if you want good framerates at decent resolution and all the graphic effects turned to medium or above, you will need something like an nvidia 1650 super or better which will mean at the very least something like a 400w PSU with a bunch of 6 or 8 pin connectors from which the GPU can draw power. And then you will still need to be sure that the GPU will physically fit in your chassis. A lot of Quadro cards for workstations are quite small so trying to crowbar a massive 3 fan high end GPU into your T5500 might be a non-starter.

If so and you find you need a more heavy duty PSU, then you may also need to investigate something like a 24 to 8 pin converter as Dell motherboards can have proprietary PSU-motherboard connections.

So sorry to give a boring story about PSUs, but before thinking about RAM, CPUs etc you need to be on solid ground on with the chassis and power. Try looking on youtube for videos by "Green PC Gamers" as they have vids on using Dell workstations as gaming machines and the kind of extra mods you will have to do.

Sorry I'm late to the party, but I'll share my thoughts. I have one of these old workstations I'm trying to retrofit into a halfway competent gaming/recording machine. This machine has several limitations that will prevent from competing with top-tier gaming machines. It's old technology that is comparatively slow compared to modern desktops. For example, the PCIe bus is version 2.0 which is half as fast as PCIe version 3.0 found in most desktops from the past year or two.

Despite such limitations, it is possible to turn the machine into a competent 1080p, maybe even 1440p gamer without breaking the bank. Since this machine was available in a number of different configurations, it would help to know what you're working with.

Upgrading to a 6-core CPU in the 3.0GHz range would be a good start (and most of them are dirt cheap!), as well as making sure your RAM is as fast as possible (1333MHz if memory serves) and in 3-channel configuration. The previous poster is right about the limitations of the power supply, though there are modern cards that will work with 2x6-pin PCIe power connectors. But the old CPUs on these old machines and slow bus speed make it likely that something like an RTX 3080 will be perpetually starved for data, and you'll never see anything close to the usual framerates for those cards. An older card like an RX 580 is a perfect fit for this old PC; it's power requirements are modest and you can probably achieve the 580's full potential with a fast 6-core CPU.

For reference, here's my configuration that I cobbled together from spare parts:

Dual X5687 @3.6GHz/36GB ECC DDR3-1333/Intel 60GB 330 SSD boot drive/640GB WD Blue/GTX 1050 Ti

Not a screamer, but it can play most modern games at medium settings @ 1080p.