Yes, you can take this in at least three different directions:
1. Try to low-ball the hardware as much as you can
2. Try to get some middle-of-the-road hardware that's currently "good" performance/price.
3. Try to get top end hardware, giving up the best performance/price ratio for performance first, and price second.
Nobody's suggesting (3) here. It's just a straw man argument.
Most are suggesting avoiding (1) here because of various reasons. I'll repeat/elaborate:
A. HL2 has very nice eye candy, why lose it to save a few $; you don't have to spend at the (3) level to get it.
B. The ancient hardware is now very poor performance / price. You should not buy really old hardware unless you absolutely don't want performance. Performance/price goes up a lot in computers over time; performance goes up, price goes down.
C. If you're playing HL2, you'll probably want to move on to other games, such as Far Cry, the newer HL2 episodes, maybe Oblivion, etc., etc. What was relevant two years ago in terms of hardware needs is not really relevant now.
I played HL2 with a 4400 Ti. That card wasn't the cheapest in its day, and gave me a lot of value over time. I also played HL2 again with a 6800 GT. I don't think I finished it, because the game's not the same the second time around. I wish I'd played the entire thing with the 6800 GT in the first place; it looked noticeably better to me. A 6600 GT can probably give you very comparable same image quality as the 6800 GT.
We all understand here that playability and image quality are two related but separate issues. We all understand that you can play HL2 at low image quality with very poor gear. The issue is why would you if you don't have to, and can get a lot more for not a lot more money. If what we're calling "not a lot more money" here is a lot of money to you, then of course, these points aren't valid -- do what you can, and enjoy the game! Of course you can do that without all the eye candy.