Heya.
There's a tool called BootConf which allows you to change the nvram settings permanently. You can get it here (right-click - save). (BootConf has issues with the [ok] button, so it might be necessary to click it twice to save.)
But your nvram battery might be empty, so the Falcon boots into low res each time you power it on. In this case you'd need to boot to TOS and use BootConf at every coldboot.

> ... 800x608, although I dont know if this can be achieved on a standard Falcon (via overscan or something maybe?)

This can easily be done using Videl inside, Videlity or CentScreen. The Videl will be used at 32 instead of 25 mhz to get this working.
I'm running one of my Falcons at 800x608 in 256 colours.
More can be done with ScreenBlaster (hardware - got 1024x768 myself when trying it) or with a graphics card like Eclipse or Nova.

Greetings, Jens