The difference would be small if the application is single threaded , but the performance gap would be huge when you're going to run multi-threaded application. For overall windows performance it would be better cause you can be ripping music from cd and your windows is still usable. You could even watch YouTube while doing that without any lags (or at least with minor ones)

my download + web server + bouncer + proxy server runs on Pentium III with 256MB of RAM . Oh it runs windows 2003 on top of linux via virtualbox. I dont complain about it , sadly it died. Now I am using my Pentium D !

Actually P4 can still play most of the games except for crazy games like GTA IV. L4D uses Source engine aka powererd by HL2. So if HL2 runs great , L4D will run great. There is nothing fancy. You can even play wow with P4 !