If you are looking for a online game then CounterStrike: Source
Half life 2 is a single player only! ( Fun game though)
You should be able to run the game on high.(very high)
Also might want to try Call of duty 4 ( which is great online fun)
There is a demo if you want to try.
I mainly want to get the game for it's multiplayer ability- which has a better multiplayer option? Half Life 2 or CounterStrike: Source? and can both games play online with people from AROUND the world?
One more thing- Here are my computer settings- Can I play both games on high/ very high settings?
CPU: Intel Pentium 4 @ 3.4GHz
Video Card: ATI HD3850 256MB
Operating system: Microsoft XP
Direct X: 9.0c