Well, for $180 you can get the 6600 GT. Comes in AGP and PCI-Express flavors, I believe, and is pretty much universally known as the best bang-for-the-buck card around. And it's equal to a Radeon X700.
And it runs HL2 pretty damn well, even with the DX9 code path.
9800 Pro, or even a 9600 XT, are also pretty good cards.
If you just want to *play* HL2, and don't care how it looks, I've seen people tweaking it so that it'll run on low-end systems. However, at that point it gets to looking like a Doom remake. You could try the demo (I think it's out) and see how well it runs.