Chosen Solution
I see that there is space for added graphics cards and appropriate slots. Is there some type of extra power cables near the graphics cards that would allow a GPU with added power requirement (6 or 8 pin power connector for card) to be added.
Depending on which Z820 power supply you have there should already be 2 or 3 6-pin connectors available for powering GPUs. The unused 6-pin connectors will be held in a plastic bracket right behind the front fan assembly. If you need 8-pin power connectors then you will need to buy adapter cables. The HP power supply provides up to 18A power on each of the 6-pin connectors, so to be safe you should buy the HP 6-pin to 8-pin adapter versus no-name ones which mostly appear to be 18AWG and can only support up to 10A. More info via the HP forums: https://h30434.www3.hp.com/t5/Business-P… HP product link: http://www8.hp.com/us/en/products/oas/pr…
You can buy any atx power supply with a 6 or 8 pin connector. A few brands I like are EVGA, Seasonic and Corsair. Luckily, they are very affordable as well. There are cables that convert molex to 6 and 8 pin connectors, but without knowing your power rating and the rails those connectors use I wouldn’t recommend them.
