I'm looking into upgrading the GPU of my 2019 rig (3700x) and I'm wondering how necessary it is to follow the manufacturer recommendations on PSU. Originally I decided not to upgrade the 8GB GTX 1080 because it simply sufficed my needs and bought a 650W PSU unit.
Fast forward to 2023 it is apparently difficult to pick a GPU which officially requires <= 650W. I'm looking for an acceptable price/performance/longevity combo and the only option right now appears to be the 4070 (not Ti). If PSU was not an issue I probably would have gone for 7900 XT instead, however paying extra 150 EUR for a new PSU on top of that is silly (let alone rewiring everything). PC Part Picker tells me I have some 70W to spare after installing a 340W Sapphire 7900 XT. I'm also limited by 330cm clearance but that's less of an issue.
It depends on the quality of your power supply. If it is from tier A or B from this list https://cultists.network/140/psu-tier-list/ I think the 7900 XT will work with no problem. But I will would look to upgrade the PSU when you upgrade your platform (CPU + Motherboard + RAM) since I believe this CPU would hold you back a little.
It's a Corsair RM650x (2018) 650 W so I'm not very concerned about the quality. It does make sense to simply upgrade the whole thing but then we're talking ~2000 EUR instead of 600-900 for the GPU alone. Choices.
I'm running a RTX4070 too and was advised to not hold back on PSU. I've completed the new built about 2 months ago and people warned me about PSU quality based on the one I had selected first. Better to invest a little more but have stable performance down the line.
NOTE: I'm also running a fairly power hungry CPU.
I ended up with a Corsair RM850x Shift. I did find installation quite a lot easier with the Shift version.
If you're willing to risk it and just try it out first with the old PSU then i would definitely try that first. I have the same PSU as you and i recently upgraded to a 6950xt. So far it's been running just fine even though AMD recommends a bigger power supply. I was also using a sapphire nitro+ vega 64 before this, which was also quite power hungry when overclocked and never had any issues. I'm not sure how much power a 7900xt draws though.
650W should be enough, unless you're getting a RTX4080 or smth. And as for my personal recommendation for a PSU manufacturer... Uuh, I've only used a PSU from deepcool and my current one from seasonic, would recommend seasonic, would not recommend deepcool...