I'm sat here with a 4090 in its box, delivered a couple of hours ago. Have my previous build which is good to go bar my PSU, it's an RM750x. Got a RM1000x coming tomorrow.
I'm itching to just throw the sucker in regardless. Only one more sleep, I'll be patient.
when i got my 4080 i was in the same situation. my GPU came before my new PSU. my old PSU was 750W too. looking back now i'd probably have been fine playing games on it but i was scared to lol. i did actually install it into my PC but i didn't play any games and set the power limit in MSI Afterburner to about 80%.
stick it in and power limit it or do an undervolt. you'd probably be fine to do a 0.05V undervolt to start off with. my 4080 is stable with a 0.1V undervolt. tried a 0.125V undervolt but it was crashing in some tests. stock it was doing 1.075V and i got it down to 0.975V. i don't know what a 4090 runs at but whatever it is just do a 0.05V undervolt and you'll likely be fine. you can run some stress tests if you want to go further. i done a mix of port royal, time spy, firestrike, heaven, and superposition. port royal for me was the one to first show instability so try focus on that if you do it.
i'm getting basically same performance and slightly higher scores compared to stock settings but with a reduction of ~4C and on average my 4080 is using less power than my 2080 in the same tests.
i know a 4090 uses more power than a 4080 but you will probably be fine to run it on a 750W psu if you set a power limit or undervolt.