Firstly, you don't seem to understand what HD really means: Your current monitor, with its 900-line display, is HD. In fact, so were your last few monitor: by modern standards, everything with more that 720 lines, like 1024x768 is HD. In the late 90s, before 'HD' became a buzzword, even 800x600 was more than a standard definition TV, and thus 'HD'.
Next, system performance is a tradeoff between framerate, effects/quality, and display resolution. If you increase any of these three factors, the others will suffer. Game designers know this, and build their software around this.
- They know that a game needs to run fairly consistently at at least 30fps.
- They know what resolutions are popular for running games
- They know what level of PC performance they are aiming at
Thus, they alter the level of effects and detail in the game so that people get reasonable performance on reasonable PCs.
What does that mean for you? Well, your PC is far above what is targeted by most game manufacturers.
- You say the framerate doesn't matter to you
- The level of detail is limited by the games themselves (more detailed models, textures and effects means more work for the developers.)
So the only way you are going to improve the 'wow' effect of your graphics are:
- Wait for a new game to come out with cool new graphics and effects
- Buy a monitor with a much higher resolution. 1920x1200 or higher, and the biggest physical size you can afford. 27" is a good price / size point.