What have RPGs have you already played? And what are your computer specs?

Baldurs Gate 1 & 2 and Fallout 1 & 2. still arguably represent the pinnacle of CRPG development. They should run on any computer and indeed, the older the better. There are literally 100s of hours of gaming in those four titles. Play them in order for best effect.

Deus Ex is an excellent RPG-shooter hybrid. System Shock 2 is even better, but hard to find legally.

A little more demanding in specs are Star Wars Knights of the Old Republic 1 & 2. They are superb cinematic single player games.

But any RPG fan will presumably have played all the above.

If your computer can handle Half-life 2, I strongly recommend Vampire Bloodlines.