Composite is very low quality to start with, it's all three colours combined rather than seperate RGB like RGB scart or component, and yes having such a low res stretched onto quite a big screen doesn't look great either. PIP should look ok though, since the PIP window is quite small (even at it's biggest it's only about 1/8th of the screen).
The 2007WFP has s-video too, so you can use that for some of those consoles. Still doesn't look great though. Tried my xbox through s-video on my 2007WFP and it looked pretty blurry for whatever reason. It must be something to do with the scaling. It's a great screen for the 360 at 1280*768 using the vga cable, but for older consoles I wouldn't like to have to use them on my Dell for any extended period. They look far better on a regular TV.