Dude, you have an Intel Pentium 4 3 Ghz! That's even faster than mine. I have an Intel Pentium 4 2,66 Ghz.
Why would you need a lightweight operating system if your computer is powerful enough to run Windows XP? Windows XP runs great on my computer, and mine is even slower than yours! My computer does also have an 40 GB hard drive and mine also has an on-board graphics card. However 512 MB RAM is quite a lot and surely enough for browsing the Internet, watching videos and listening to music, your computer will perform better if you have more RAM. My computer has 1014 MiB RAM.
I recommend Windows XP. If you want to have a lighter operating system, I would go for Windows 2000 Professional. If you are running Windows 2000 Professional, I recommend to update immediately to Service Pack 4 and to install all available updates. Because of the fact that the most software these days doesn't support Windows 2000 Professional anymore, you need older versions of some programs. Then you need Opera 11.64, Mozilla Firefox 12, VLC Media Player 1.1.9, Windows Media Player 9, Internet Explorer 6, WinRAR 4.11 (or the latest version of 7-zip), Java 6 update 24, Adobe Flash Player 11.1.102.55 and Acrobat Reader 9.5. Avast! Antivirus will run on Windows 2000 Professional.
If Windows 2000 Professional isn't lightweight enough, Windows 95 or 98 would be an option. However Windows 9x is pretty unstable and you will need even older versions of programs.
Puppy Linux is an option, but note that Puppy Linux has heavier system requirements than Windows XP. Windows XP is lighter because it doesn't take that much RAM as Puppy Linux. Windows XP can run on an Intel Pentium 233 Mhz and Puppy Linux can't.
From my experience I can tell you that Windows XP and Puppy Linux are great operating systems. I have two operating systems installed in this computer: Windows XP and Puppy Linux. They run great. I like both of them, however I mostly use Windows XP because Windows XP is compatible with the games I play.