i say 4.0ghz. but i dont know. your sh**ty ram could inhibit you. or, if your mobo is so sh**ty, it could hold you back. your sh**ty stock psu may be a problem as well. hopefully your p4 isn't so sh**ty as you say.
if you dont care about it, overvolt it, crank up the clock, and watch it melt. then, buy a computer that is not so sh**ty.