I have a question about upgrading a graphics card.

[Diamantius]

Hi SDH,
Thanks so much for taking the time to reply. I read everything I could find on the forums regarding this issue. There is one thing I don't understand, though: Would "standard" 6-pin to 8-pin, or 6-pin to 6-pin+8-pin adaptors work? Or do I need to get the specific HP parts? These are crazy expensive, from what I see.

I am thinking of going with the 1660 Ti, which has an 8-pin connector but only 120 W TDP. Would an adaptor like this one work?
https://www.ebay.co.uk/itm/2-pack-10cm-PCI-Express-PCIe-6-Pin-to-8-Pin-Graphics-Card-Power-Adapter-C...

[roison]

My RTX 2070 is a short-length edition card, which with only one 8pin power socket(Supplementary Power Connectors).

http://www.inno3d.com/products_detail.php?refid=397

So I used a  6pin to 8pin adapter cable. It has been working fine for 6 months.

In my opinion, It's should be OK with other RTX2070 editions with double power sockets (6pin and 8pin commonly?), using one 6pin to double 6+2pin adapter cable (see image below), as SDH said. There is no RTX2070 card needing a power assumption of more than 291W(75W+216W).

PS: Now the best cost-performance trade-off choice is RTX 2060 Super (RTX2060s).

[Diamantius]

Thanks for the clarification. So standard adapters should work just fine, since power consumption of these cards is relatively low. 
Budget wise, I am closer to the 1660 Ti, or maybe the 2060 6GB if I stretch it a bit. 

[SDH]

HP engineers would know best about wiring for maximum wattage through their non-ATX use of the 6-wire cable(s) carrying more than usual wattage from the HP power supply to a video card having an 8-pin adapter built in.  As such it is ideal to use the 6-to-8 wire adapter they made.  These are somewhat hard to find and not cheap.  My advice on this is to get one of those if possible, and to get a brand name adapter that has as many of the pins populated as possible such as from StarTech.  Here are some of the HP part numbers for the 6-to-8 adapter:

Older HP version:  460621-002

Newer HP version:  683867-001

N1G35AA

Take a look at the HP ones above via Google and note they have most of their lines populated.  Some no-name junkers have few wires.