If you are doing it via CLI, as long as you do a 'write mem' or 'copy run start' before rebooting, you won't lose any config.
This is correct – the configurations are stored in Non-Volatile RAM (NVRAM) and the IOS image is stored in flash (so you can erase the flash if you want to without losing your configuration – just make sure you put an image back on before reloading!).
The command you'll need to use is something like this (all on one line):
Switch# archive download-sw /overwrite /reload tftp:[[//location]/directory]/image-name.tar
Full command details here:
http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_25_sea/command/reference/cli1.html#wp2273183