Just so put it out there.
HL2 Specs.
Minimum: 1.2 GHz Processor, 256MB RAM, DirectX 7 level graphics card, Windows 2000/XP, Mouse, Keyboard, Internet Connection
Recommended: 2.4 GHz Processor, 512MB RAM, DirectX 9 level graphics card, Windows 2000/XP, Mouse, Keyboard, Internet Connection
I was running it on my AMD 2400+ XP with 512MB Ram and Geforce 4 MX440. Didn't look great, but it ran fairly smooth. RAM is dirt cheap now anyway, so you should invest in some if you use your computer for more than surfing the web.
Edit: Here are the orange box specs.
1.7 GHz Pentium 4
512 MB system RAM
DirectX 8 compatible video card