Well, for a while game makers were being sensible and wanted to make money, so they catered to the average system.
Now, game makers are catering to systems much higher. Supreme Commander for example pushes the limits on systems that aren't even made yet in some instances. I can get it to work on some systems older than a year or two, but it highly depends on the specs. (The game delivers everything they said it would by the way, but the middle to end game is a bit overwhelming with all that you have to pay attention to).
On the otherhand your system should be pretty good for some of the hotter games coming out. For example I'm certain it runs WoW and it's expansion the Burning Crusade excessively well. It probably will also run the new Command and Conquer game that's going to be coming out soon (Command and Conquer 3 the Tiberium Wars), and run it better than most computers out there.
It's a good computer, just some gamemakers create their games depending on the specs of a console such as the Xbox 360 without a care of how it might run on a computer, and thence don't really consider that to actually sell games they should cater to a majority who DON'T have the most up to date and current rig.
That's why Blizzard repeatedly sells so well, they create great games.......
That actually can run on a majority of people's computers.