Common wisdom is to create a file /etc/grub.d/01_915resolution
with the appropriate content, e.g.:

echo insmod 915resolution
echo 915resolution 54 1024 600

However, nowadays, /etc/grub.d/00_header already initialises
the graphics mode, so this is too late.

I’m trying (haven’t yet rebooted) to move it to 00_915resolution,
but anything coming before a file called 00_header may be suspect
(and, in fact, the 915resolution module may be not even accessible
by that point), and creating a file myself to run 915resolution
also is… not exactly declarative, so a better integration for such
things would probably be welcome by many users?

(Of course it’d be even better if those eeepc had a standardised
resolution, but… it was a gift, so I better not complain.)