Which graphics card exactly? There's a good chance that's struggling as well.
As someone else already said, you *will* need to run with low res textures with only 256MB.
To overclock a P4, you'd probably have to do it in the BIOS (likely by increasing the multiplier if it'll let you). Be careful though, you can easily fry it if you take it too far - small increments and do a stress test every time, keeping an eye on temps.
If you want to play it (slightly) safer and ebay, there were some 3.0GHz P4s @200MHz FSB. Check compatibility first though.
Oh, and you really don't need an i3 and dedicated graphics to run LFS - I've had a £70 AMD APU (CPU with integrated graphics) running the new Westhill at well over 100FPS on a 1440x900 screen. You'd probably even get away with the £50 one for 60+FPS at that kind of resolution.