Buildapc

How bad is it to trigger the overdraw protection in the PSU? Obviously my PC would shut down immediately (with all that entails), but would I risk damage to components?

I suppose theoretically low voltage could damage something, but I doubt that it's very likely.

I'd probably favor getting the PSU, because otherwise, if you hit any stability issue, you're going to be thinking "is it the PSU"? But I wouldn't be too worried about component damage.

I'm set on the 7900XTX, as I want the 24GB VRAM. It's quite literally the cheapest option (new, in my region) that is also a usable gaming card.

Note that if you're wanting local AI compute capacity

the likely reason to want a lot of VRAM attached to a GPU other than gaming

and you can live with less memory bandwidth and a substantial price bump isn't an issue (which it sounds like it is, but just throwing this out there), you might also consider a unified memory computer, like a Framework Desktop, as that can take a lot more memory than a consumer GPU. I have a 24 GB XT 7900 XTX and a 128 GB Framework Desktop, and I've found myself using the latter rather than the former, especially for LLM stuff.

If you're willing to wait until 2028, my expectation is also that memory prices will be better and thus bang-for-buck on local AI compute will also be better. Also give the hardware guys time to iterate on hardware; probably improve performance.

I don't know about LACT cause i haven't used it, but if you have a good quality psu (and it looks like you have) it should be fine. I was in a similar situation when i went from a vega 64 to a 6950xt. I had an RM650x but 750w was recommened for the 6950xt. I just tried using it without upgrading the psu and for gaming it seemed to work fine. It wasn't until i started messing around with genAI that the psu would turn off my system. I'm assuming the AI stuff hit a power spike that gaming workloads did not. I eventually upgraded to a 1000w psu, but i was able to get by for a bit with the 650w.

You should be ok, even if you touch 750W every now and then. Corsair PSUs are generally decent and can likely hold over 750W for short periods. What you are sacrificing is PSU efficiency, but if it's temporary, whatevers. Don't overclock the CPU and under volt the GPU and don't run heavy and continuous compute loads for long periods of time.

While the power management is likely a hair different with my flavor of 7900XTX, it has always behaved well under heavy load and benchmarking and it rarely spikes to max power unless all the OC knobs are pushed to max.

Unfortunately, I can't speak to how the 7900XTX is managed under Linux. If I am not mistaken, unless you are purposely overriding power profiles, you should be fine with whatever the stock settings are on the card itself. (7900XTX's do come in non-OC and OC variants or have a switch for a dual bios for either configuration. Manufacturer OC settings can still be quite timid, IMHO.)

Depending on your specific components, the system may crash before it hits a power limit in some cases. If the PSU can't handle the power draw requirements, the power rails might sag a little during a burst and crash the system. I have only seen this a couple of times, but it's still worth mentioning.

Ignoring all of the above, just run some tests under different power loads, and in this case, aim for the lowest power settings you can, just to be on the safe side. While not perfect, invest $20 in a kill-a-watt power draw monitor for system testing. At a minimum, you will get a rough idea of total system power draw, which is good enough here. (I have seen ~+/-5% skew between different kill-a-watts, which isn't an issue unless you are seriously working against a strict draw limit.)

I may be able to provide some personal experience regarding the first part of the first question, "How reliable is the Power Usage Limit I set in LACT enforced?". The answer appears to be application based and I will do my best to explain.

I was having a persistent issue with my GPU PPT0 throttling during gaming sessions with a specific area in a specific game. This was the first time I had ever experienced this odd behavior which initially presented as a rapid increase in GPU fan speeds and a small loss in frame-rate. By using LACT, I was able to confirm why this was happening and the real-time data was indicating that as soon as the GPU got within about 10% of the power cap, it would start to power throttle. The crazy thing was, the 170 Watt cap would not stop the card from drawing over 190W in short bursts. By increasing the power cap to 190W, it did reduce the throttling somewhat but then my GPU was still exceeding this revised cap and I didn't see any real value in pushing the value higher.

I have a decent safety margin with a 9600X and a 9060XT using a Corsair 750e PSU. I imagine your setup has much higher peak power needs so I would suggest reducing the power cap by 10% to start and then incrementally adjusting the core voltage downwards using the undervolt option. This should allow you to achieve a stable operating level if you are careful and find the sweet spot between performance and power draw. Otherwise, getting a beefier PSU may be your safest bet.