have a look on gog.com for some good games for a good price. i had a pc with similar specs and some games i enjoyed were (with personal ratings
)
the original mass effect - action- 11/10,
company of heroes -RTS- 10/10,
world in conflict -RTS-10/10,
heroes of might & magic V -RPG- 6/10
dragon age origins -RPG-10/10,
dawn of war -RTS-9/10,
fallout 3 -FPS/RPG-10/10,
the evergreen Warcraft 3 -RTS- 10/10,
Starcraft 2 would probably run quite well -RTS- 8/10,
Red Alert 3 -RTS- 7/10,
call of duty: modern warfare -FPS- 8/10,
Borderlands -RPG- 9/10,
these are some of my favorite games, i spent way too many hours on these titles!
you should find something among these fine games.