I'm running on a pretty low-end PC and I'm looking to play some games that will keep me pleased, even with the results I'm getting. My PC can run Warhammer 40K : Soulstorm etc pretty effectively.. put it that way. Specifically any RPG's would be great because they normally have a lot of replay-ability.
@razorzxz: I`m actually running Deus Ex: Human Revolution on a single-core Compaq Presario with all the settings at minimum and it`s still a gorgeous game! I have GameBooster 3 which cuts out all unnecessary apps while playing. (It`s available at IOBit`s website for free)! And yes I consider the game an RPG.
If you're not looking for a swords-and-magic type RPG, Fallout 3 and Fallout: New Vegas should run pretty well on your system.
If you're into some multiplayer, you can download Team Fortress 2 in Steam for free.
Also, Portal and Portal 2 if you haven't played them.
Any Fallout game
Dungeon Siege 2
Witcher (although you might wanna go over to this site, considering that you didn't give us your specs :P)
I'd go with the Fallout games if you haven't played them. Great universe, all of them have great gameplay, atmopshere and story and lots of replayability. Many people suggest Torchlight and it is a really fun game but it literally has a paper thin story, so it's up to you.