RPGs that aren't top-down and will run on your system... that's a tricky one. KOTOR and Jade Empire might work. I saw mentions of The Witcher, Oblivion and Bloodlines and I doubt any of them would run. You could look into Arx Fatalis. I own it, but have never played it. Can be found on Steam and gog.com for cheap. Older Elder Scrolls games - such as Daggerfall (freeware, I think) - will be an option. If you don't mind something like Diablo then check out Torchlight as well.
Non RPG games, look up Starlancer for a great space combat game (probably hard to find though). Adventure games will be your friend as they're pretty low spec, so I'd say to check out The Longest Journey. Impulse has an offer on Total Annihilation this weekend which is always a classic RTS. And speaking of classic RTS, various Command and Conquer games are freeware available on the EA website. For lots of things that will almost certainly run, I highly recommend you check out Good Old Games,