My PC is pentium 4, 2.4 ghz amd athlon.
What should I overclock it to so that it is faster and wont cause significant risks ?
Thanks!

If your pc is frustrating you because of its speed, then I suggest you to buy a new(er) one
For the challenges on this site it doesn't matter that much. Ofcourse it will take a little longer, but brute force challenges don't takes hours here (if we ignore the challenge 'Flash 3').

If you have some decent OC software, it should show that.
Else, I would suggest not going above +10% to be "safe".

depends how bad your algorithm is

PS: If it's not broken, don't try to fix it .