Yep, it seems it's an actual design flaw in the GeForce FX cards. I wish somebody would re-examine it with the later rev 5x50 series of GeForces though. To see if they still suck.
As for the original question: Half Life 2 will run fine on everything from a Vodoo 3 up to a Radeon 9800 XT. It supports DirectX 6, 7, 8, and 9, and selectively turns stuff off to qualify for the lower DirectX specs. For comparison, the original Half-Life was DirectX 6. The Radeon 9600 Pro is a DirectX 9 card, and will run the game just fine. And if you find it runs too slow, scaling back to a previous DirectX spec will net you LOADS of speed.
As for processor, the Alienware is more than fast enough. Remember, Half-Life 2 is designed to run on everything from computers that run the original "Well" to computers that are top of the line.