Jedi Outcast worked on all of the PCs I've owned including my old Pentium 4. I would highly suggest it as it is still unbelievably fun to force throw stormtoopers into the ground and full dismemberment is possible with your lightsaber (if you know how to add it in the console) Also the narrative is really well done for single player.
Academy is good if you like options in your single player and want to play it in any sequence you want. (I hated heck out of the story, but maybe you might dig it)-If you're into it, full dismemberment is hard to apply, but with time and google, I was able to get it to work-
Half-Life 2 is one of my absolute favorites and it is still a graphics tour de force to this day. I like physics gameplay and you really can play it however you want with the grav gun. The script is also top-notch and the sound quality is quite nice.