Overclocking a dell u2414h help please

[ToastyX]

HDMI is the same as single-link DVI. You can't convert HDMI or DVI to DisplayPort with a passive cable. You need a video card with DisplayPort.

You didn't mention what video card you have. AMD cards require the "LCD standard" vertical blanking/total to reduce the memory clock when idle, but the monitor might not be able to handle that at higher refresh rates. NVIDIA cards can handle some lower values, but not too low.