|CPU-Z is not to overclock, but to check your settings.|
Your Dell machine like nearly all manufactured systems had the BIOS settings locked that might let you overclock so there is no path there. Software overclocking is possible, but the amount you might get is not worth the bother. The reason you BIOS is locked is because the system is a whole bunch of compromises in quality, speed, and price so the hardware they select is not overbuilt in the way you would need it to be to support the extra stresses that overclocking puts on the system components.
The problem with the P4 systems today is not just the CPU speed, the fact that it is a single core, or even the memory speed, it is the complete package. The memory band width, the buss 'width' and speed, and other factors make it no longer a viable choice for today.
If you purchase an inexpensive desktop, or even a laptop that has a 'slower' rated ('speed') but modern CPU, that system would blow away a Pentium 4 system even if it was custom built with the highest possible quality components and overclocked to the limits. There are even some tablets and a couple of smart phones that would be faster on many things than a system that old.
Your best bet it to purchase or build a new system that is the best you can afford to be as future proof as possible.
You have to be a little bit crazy to keep you from going insane.