Plaaaaaaanescaaaaaaaaaape toooooormeeeeeeeeeent.
Really now.
Oh, besides that? Lessee... Sacrifice. On Steam, unsure about specs. System Shock 2, not on Steam, still excellent. X-COM, entire series is on Steam, ignore interceptor/enforcer.
Hm... older Thief games? And the Avernum/Geneforge series, while not old or on Steam, are fairly old game-y in style and graphics, so they should run fine. Quite good indie RPGs.
Edit: And every roguelike ever. Seriously, binge on them. I personally suggest Nethack and Dungeon Crawl Stone Soup.
Again, not old, but excellent and something your computer can handle: Iji. You owe it to yourself to play this, much like Planescape.