You definately want a GForce 3 or better then.
Half-Life uses hardware accelerated T&L (Texture and Lighting), and since lots of processing is done on the card itself, it is less painful to your main CPU. The GForce3 I believe does a hell of a lot of other things on the board itself, this is all good news for you.
And definately get a 64Mb version of you can afford it. A GForce3/4 Ti200 is fine, you don't need the top-spec models, they cost about twice the price for very little performance increase.