It's a significant boost.
Following the reveal of Gamecube games being added to the Nintendo Switch Online service for Switch 2, Nintendo showed several brief trailers for the titles launching on the platform. After counting resolution on each title we are able to confirm that Gamecube titles on Switch 2 appear to be running at 900p (1350 x 900). this is a near doubling of their original resolution of 480p or 480i depending on your connection type. Previously on the original Switch, Nintendo 64 games ran at 720p.
The Switch 2 is capable out outputting up to 3840 x 2160 so while this is the highest resolution we've seen from a NSO service title, it is well below the maximum output of the system.