According to Valve's list of requirements for Half-Life 2, a DirectX 7 level card is needed to run the game. While Nvidia's 8400 GPU is not geared toward gaming it will run Half-Life 2 fairly decently at medium graphics settings.
Some games may not run as smoothly on the nvidia 8400. I would recommend getting an 8500 if you have the money. If games do not run smoothly you can always lower the res and graphics of the game when you're playing.
If you want to know how good your video card is, just download and run the game. If the screen does not load, blacks out, or lags than you should invest in a different video card to play games.
Yes! The graphic card you are using is still very good for gaming. My family uses it in our gaming computer. Just be sure to update regularly for best performance.
The nvidia 8400 would be very good for gaming. It could definitely run Half-Life 2 at full settings and could also play newer games like Call of Duty 4.
