Do as I did....go to google and look for " Best PC games of 2007 ".
Check them out, and if they are still too much to handle for your PC , go back a step and look for best games of 2006, 2005, 2004.....
When you reach the sweet point ( games that run smoothly on your hardware ) try to look for more popular games of that year.
But let's say your computer is a little outdated, if it can play DOOM 3 BGF or the original decently, then you could go for best games of 2004.
If on the other hand you have a weak CPU and an integrated videocard ( not very good for games rendering ) then stick to late 90's, early 2000 and you should be fine.