Delver is a first person randomized ARPG with an emphasis on dungeon crawling. It's extremely easy to run. My netbook handles it maxed with no problems. Not free though.
http://store.steampowered.com/app/249630/
There's also TES: Daggerfall. In the tradition of Elder Scrolls games, it's first person and open world. It went free a couple years back when Skyrim released, and Bethesda now hosts a free download for it on their official site. It's completely compatible with DOSbox. My netbook has some framerate slowdowns in parts of the game, but it's actually weaker than your current PC, so I doubt you'll have problems. Ofc my desktop handles it fine.
http://www.elderscrolls.com/daggerfall/
Drakan: Order of the Flame is an old action-based somewhat open-world RPG. Not first person unfortunately, but I think it's classified as abandonware, as the company that made it collapsed and there was recently a fan 'remake' of sorts available for free.
http://www.drakan.co.nr/
Enclave is an old single player RPG from like 2001 or 2003. Seeing as it's really old, it can't be too hard to run. Great level design, but it's not open world. It has options for a first person and third person camera, which can be toggled with the "G" key, but third person is more polished. It's not free, but it is on sale on Steam right now for $1.
http://store.steampowered.com/app/253980/
Arx Fatalis is a semi-open-world first person single player RPG. It has some compatibility issues with modern hardware, but there's a fan patch/remake/installer called Arx Libertatis to fix those issues, and the company that made it patched in Windows 7 support on the 10th anniversary. It's not free, but it is cheap ($5) on Steam.
http://store.steampowered.com/app/1700/
http://arx-libertatis.org/