Pigtails

Hey, recently I've upgraded to RTX 3080 (From 1080) and I've used pigtail 8pin to get the job done, even for the RTX 3080, but I noticed it only draws 280 power instead of ~330, so I got the second cable from the box and replaced the pigtail with the standalone 8pin's and the PC won't turn on...

I'm a first time PC builder. I'm looking at building a gaming rig for approximately £1500 but wouldn't mind spending little more if it makes it a lot better. Do people think there is much difference between the Radeon R9 295X2 and Dual GTX 970s? This is my idea for a Radeon rig...