A return to PC gaming

I have just recently bought myself a fresh mid-range graphics card and thus making my PC gaming capable without serious eye sores. The main reason for this purchase at first was to play Civ 5 on launch but then I started thinking why limit myself to just Civ, I have yet to experience the gaming goodness PC has had to offer these past 5 years or so. Somehow I'm still stuck in that mind frame of thinking that PC gaming is too expensive for me but looking through Steam on my newly created account, yeah I know I'm so with the times, I realize that my almost 4 year old Packard Bell pre-built box with an updated graphics interface is capable of running most of these games. So I set out to try some demos to see for myself how the PC could handle various games. So far Mass Effect 2 and Mafia 2 are running good in full resolution 1440x900 at acceptable frame rates and quality except for the occasional hiccup in Mafia.  
Now, I know that ME2 and Mafia2 are not graphical powerhouses but they do need acceptable specs to run decently and I guess I never imagined that I could keep my PC gaming capable for a relatively low  investment. I have now seen the error of my ways and I'm even thinking of upgrading more components to be able to run stuff more smoothly, mainly Civ 5 which is more demanding on the CPU and RAM than the GPU.  
So next up is exploring the back catalog of PC and multi-platform games I have missed. Now I need you Giant Bomb users to tell me which games to play. I have already decided on three PC game purchases that will keep me busy for a long time which are the original Mass Effect, Portal and Civilization 5. 
I can pretty much play all genres but I guess I'm mostly interested in games that offer something original and/or have a good story.