There are a few games on here that I am unsure of & a demo would go a long way towards helping me decide if a game is worth the purchase (like Psychonauts & Beyond Good & Evil for example). So is there any chance that you guys will be putting demos up for download?

Although I like the idea of GOG providing demos, it's very unlikely to happen, I would guess. There are lots of demos out there though (even for Psychonauts and Beyond Good and Evil), just google it.

Community once compiled a list with demos of GOG games. Although a bit outdated, it can be found here:
http://www.gog.com/en/forum/general/demo_links_for_current_gog_games/page1