You should be able to play most newer games on minimum settings. And all games until 2005 should run decent.
Here some examples from rather actual games:
For example Mass Effect 1 minimum requirements:
Singlecore-CPU 2,4 GHz, 1024 MB RAM, 128 MB Graka, DirectX 9.0c, 6.5 GB
Mass Effect 2
Dualcore-CPU 1,8 GHz, 1024 MB RAM Windows XP or 2048 MB RAM Windows Vista/7 , Geforce 6800 or Radeon X1600 Graka 256 MB RAM, DirectX 9.0c, 15 GB
You should be able to play some games atleast on lowest settings.
Oblivion:
512 MB system RAM
2.0 GHz processor
128 MB video card
8x DVD-ROM drive
4.6 GB hard disk space
Here some older decent games which will run perfectly:
Diablo 2
WOW
Warcraft 3
Doom 3
Command and Conquer 4
Half Life 1
Half Life 2